it; 


THE 

METALLOGRAPHY   OF   IRON 
AND    STEEL 


BY 

ALBERT   SAUVEUR 

i  < 

Professor  of  Metallurgy  and  Metallography  in  Harvard  University 


FIRST   EDITION  — FIRST    THOUSAND 


McG  RAW-HILL  BOOK  COMPANY 

239  WEST  39TH  STREET,   NEW  YORK 

6  BOUVERIE  STREET,  LONDON,  B.C. 

1912 


.    ,3 


COPYRIGHT,  1912,  BT 
SAUVEUR  AND  BOYLSTON 


THE  UNIVERSITY   PRESS,   CAMBRIDGE,   D.  8.A. 


TO 

THE    MEMORY    OF 

4Hp  Jfatfter 

I  REVERENTLY  AND  LOVINGLY 
DEDICATE  THIS  BOOK 


PREFACE 

WHILE  several  excellent  books  on  metallography  have  been  published  and  while 
numerous  papers  on  the  metallography  of  iron  and  steel  have  appeared  in  the 
scientific  and  technical  press,  a  well-balanced,  specific,  and  comprehensive  treatise 
on  the  subject  has  not  heretofore  been  written.  In  the  belief  that  there  is  a  real  and 
urgent  need  of  such  a  treatise  the  author  has  endeavored  to  supply  it,  craving  for  his 
effort  the  indulgent  criticism  of  his  readers.  He  offers  his  book  to  those  seeking  self- 
instruction  in  the  metallography  of  iron  and  steel,  their  special  needs  having  been 
carefully  considered  in  the  arrangement  of  the  lessons;  he  offers  it  to  teachers  and 
students  trusting  that  they  will  find  it  valuable  and  suggestive  as  a  text-book;  he 
offers  it  to  manufacturers  and  users  of  iron  and  steel  in  the  belief  that  he  has  given 
due  weight  to  the  practical  side  of  the  subject  and  has  avoided  discussions  of  ill- 
founded  or  purely  speculative  theories;  he  offers  it  to  the  general  reader  interested 
in  the  scientific  or  practical  features  of  the  metallography  of  iron  and  steel,  as  the 
language  used  should  be  readily  understood  by  those  lacking  specialized  knowledge 
of  the  subject;  he  offers  it  to  experts  in  the  hope  that  they  will  find  it  not  entirely 
devoid  of  original  thought,  original  treatment,  and  suggestiveness. 

In  the  matter  of  illustrations  and  especially  of  photomicrographs  the  author's  aim 
has  been  to  utilize  the  best  available,  using  his  own  or  those  taken  in  his  laboratory 
only  when  no  better  ones  have,  to  his  knowledge,  been  published  by  others.  The 
original  source  of  every  illustration  has  been  indicated  and  the  author  desires  to  ex- 
press his  indebtedness  to  the  following  writers,  the  figures  in  parenthesis  showing  the 
number  of  illustrations  from  each:  Andrews  (3),  Arnold  (7),  Bayley  (1),  Belaiew  (5), 
Brearley  (2),  Carpenter  and  Keeling  (l),Sherard  Cowper-Coles  (1),  Desch  (4),  Edwards 
(2),  Ewing  and  Rosenhain  (2),  Guillet  (18),  Gcerens  (9),  Gulliver  (2),  Hall  (1), 
Houghton  (1),  Kroll  (1),  Law  (8),  Levy  (1),  Longmuir  (2),  Matweieff  (1),  Maurer  (1), 
Mellor  (1),  Osmond  (17),  Roberts-Austen  (1),  Robin  (1),  Roland-Gosselin  (1),  Rosen- 
hain (2),  Saladin  (2),  Sorby  (1),  Stead  (13),  Tschermak  (3),  Tschernoff  (1),  Wiist  (5), 
Ziegler  (1).  All  illustrations  not  otherwise  inscribed  are  the  author's. 

The  author  cannot  refrain  from  expressing  here  the  sorrow  and  sense  of  personal 
loss  he  experienced  when  the  news  was  received,  while  this  book  was  passing  through 
the  press,  of  the  death  of  Floris  Osmond,  for  to  the  author,  as  no  doubt  to  many  others, 
Osmond's  work  and  Osmond's  life  have  been  an  inspiration.  Osmond  belonged  to 
that  admirable  class  of  French  scientists,  who,  like  Pasteur  and  Berthelot,  have  so 
lofty  a  conception  of  the  duty  of  the  scientist  that  they  give  to  the  world  the  fruit  of 


VI  PREFACE 

their  genius  and  of  their  untiring  labors  with  no  thought  of  monetary  return  or  even 
of  honorary  recognition.  If  Sorby  was  the  pioneer  of  metallography  and  Tschernoff 
its  father,  Osmond  has  been  its  torch-bearer  for  he,  more  than  any  other,  has  been  our 
guide.  While  he  is  no  longer  with  us,  his  light  will  long  continue  to  burn  and  to  show 
the  way  to  promising  and  productive  fields  of  research. 

The  author  desires  to  place  on  record  his  warm  appreciation  of  the  assistance  he 
received  from  Mr.  H.  M.  Boylston  in  passing  this  book  through  the  press,  and  also 

for  many  valuable  suggestions. 

ALBERT  SAUVEUR. 

HARVARD  UNIVERSITY, 

CAMBRIDGE,  MASSACHUSETTS, 
August  19,  1912. 


TABLE    OF   CONTENTS 

INTRODUCTION 

PAGE 

THE  INDUSTRIAL  IMPORTANCE  OF  METALLOGRAPHY 1 

APPARATUS   FOR  THE   METALLOGRAPHIC   LABORATORY 

THE  MICROSCOPE 1 

The  stage 1 

Plain  stages 3 

Mechanical  stages 3 

Objectives 3 

Eye-pieces 3 

Iris  diaphragms 7 

Specimen  holders 7    j 

UNIVERSAL  METALLOSCOPE 10 

Electromagnetic  stage 11 

Templets  for  the  examination  of  small  specimens     11 

Support  of  non-magnetic  specimens 11 

Leveling-devices  of  stand  and  stage 12 

Motion  of  the  stage 12 

Mechanical  stage 12 

Examination  of  transparent  objects 13 

ILLUMINATION  OF  THE  SAMPLES 14 

SOURCES  OF  LIGHT  AND  CONDENSERS      18 

Monochromatic  light 22 

PHOTOMICROGRAPHIC  CAMERAS 22 

INVERTED  MICROSCOPES 28 

POLISHING  APPARATUS 28 

Hand  polishing 28 

Polishing  by  power 30 

PYROMETERS  AND  ELECTRIC  FURNACES 30 

Pyrometers 30 

Electric  Furnaces 35 

LESSON  I  —  PURE   METALS 

Microstructure 1 

Crystallization      1 

Idiomorphic  crystals 2 

Allot  rimorphic  crystals 2 

Crystallization  of  metals 2 

Grains  of  metals 

Crystalline  orientation  of  the  grains 3 

Cubic  crystallization  of  metals.  —  Etching  pits 

Summary 

Impurities " 

Influence  of  thermal  treatment 7 

Influence  of  mechanical  treatment ° 

Examination 8 

vii 


viii  TABLE  OF  CONTEXTS 

LESSON  II  — PURE   IRON 

PAGE 

Microstructure 1 

Cubic  crystallization  of  iron 2 

Ferrite 4 

Allotropy  of  iron 4 

Influence  of  impurities 10 

Influence  of  heat  treatment - 10 

Influence  of  mechanical  treatment 11 

Straining  of  iron.  —  Slip  bands 11 

Examination 12 

LESSON  III  — WROUGHT   IRON 

Chemical  composition 1 

Microstructure  of  longitudinal  section 1 

Microstructure  of  transverse  section 2 

Chemical  composition  of  slag 3 

Microstructure  of  slag        3 

Influence  of  thermal  and  mechanical  treatments 4 

Experiments 4 

Polishing  by  hand 4 

Polishing  by  power 6 

Etching 6 

Etching  with  picric  acid 6 

Examination 7 

Etching  with  diluted  nitric  acid 7 

Etching  with  concentrated  nitric  acid      8 

Examination 8 

LESSON  IV  —  LOW   CARBON   STEEL 

Normal  structure 1 

Grading  of  steel  vs.  carbon  content      1 

Low  carbon  steel  vs.  wrought  iron 1 

The  structure  of  low  carbon  steel 2 

Pearlite 3 

Free  ferrite 4 

Cementite 5 

Experiments 5 

Polishing 5 

Etching      6 

Photomicrography 6 

Exposure 7 

Diaphragms  and  shutters 7 

Monochromatic  light 7 

Photographic  plates 8 

Development 8 

Printing 8 

Mounting 8 

Examination .• 8 

LESSON  V  —  MEDIUM   HIGH   AND   HIGH   CARBON   STEEL 

Medium  high  carbon  steel      1 

High  carbon  steel 4 

,/^Eutectoid  steel 4 


TABLE  OF  CONTENTS  ix 

FACE 

Hyper-eutectoid  steel 4 

Free  cementite      5 

Hypo-  vs.  hyper-eutectoid  steel 6 

Etching  of  cementite 7 

Carbon  content  of  pearlite 8 

Structural  composition  of  steel      8 

Chemical  vs.  structural  composition 11 

Micro-test  for  determination  of  carbon  in  steel      12 

Physical  properties  of  the  constituents  of  steel 14 

Tenacity  of  steel  vs.  its  structural  composition      15 

Steel  of  maximum  strength 17 

Ductility  of  steel  vs.  its  structural  composition 17 

Diagram  showing  the  relation  between  the  tenacity  and  ductility  of  steel  and  its  carbon  content  19 

Experiments 19 

Etching      19 

Etching  with  sodium  picrate 19 

Photomicrography 19 

Examination 20 


LESSON  VI  —  IMPURITIES   IN   STEEL 

Metallic  impurities 1 

Xon-metallic  or  oxidized  impurities 1 

Metallic  vs.  non-metallic  impurities 1 

Gaseous  impurities 1 

Impurities  vs.  physical  properties  of  steel 1 

Silicon  in  steel .• 1 

Phosphorus  in  steel      2 

Sulphur  in  steel 2 

Manganese  in  steel 5 

Chemical  vs.  structural  composition 6 

Xon-metallic  or  oxidized  impurities 8 

Gaseous  impurities 9 

Segregation  of  impurities.  —  Ghosts 10 

Experiments      11 

High  vs.  low  phosphorus  steel 11 

High  sulphur  steel 12 

Oxidized  Bessemer  metal 12 

Segregated  steel -.    .  12 

Examination 12 

LESSON  VII  —  THE   THERMAL   CRITICAL  POINTS   OF  IRON   AND   STEEL 

THEIR  OCCURRENCE 

Point  of  recalescence 1 

Notation 2 

Critical  range.  —  Transformation  range 2 

Position  of  Ari  and  Aci 2 

Speed  of  cooling  and  heating  vs.  position  of  AI      4 

Chemical  composition  vs.  position  of  Ai 5 

Upper  critical  points 5 

Thermal  critical  points  in  pure  iron 5 

Thermal  critical  points  in  very  low  carbon  steel 6 

Peculiarities  of  the  point  A2 6 

Thermal  critical  points  of  medium  high  carbon  steel 6 

Merging  of  A3  and  A2      .6 

Thermal  critical  points  in  eutectoid  steel 7 


x  TABLE  OF  CONTENTS 

PAGE 

Merging  of  A3.2  and  AI 7 

Thermal  critical  points  in  hyper-eutectoid  steel 7 

Merging  of  A3.2.i  and  Acm 8 

Minor  critical  points 8 

Data  showing  the  position  of  the  critical  points 8 

Relative  quantities  of  heat  evolved  or  absorbed  at  the  critical  points 8 

Graphical  representation  of  the  position  and  magnitude  of  the  critical  points 10 

Determination  of  the  thermal  critical  points 10 

Cooling  and  heating  curves 10 

Use  of  neutral  bodies 14 

Additional  illustrations  of  cooling  curves 18 

Self-recording  pyrometers 18 

Historical 19 

Experiments     If 

Examination 20 

LESSON  VIII  — THE   THERMAL   CRITICAL   POINTS   OF  IRON   AND   STEEL 

THEIR  CAUSES 

Causes  of  the  upper  points  A3  and  A2  in  carbonless  iron 1 

Causes  of  the  upper  critical  points  A3  and  A2  in  low  carbon  steel 3 

Cause  of  the  point  A3.2 4 

Cause  of  the  point  AI 4 

The  point  At  an  allotropic  point 6 

Pearlite  formation 7 

Cause  of  the  point  Acm 7 

Allotropy  of  cementite 9 

Cause  of  the  point  A3.2.i  in  eutectoid  steel      9 

Cause  of  the  point  A3.2.i  in  hyper-eutectoid  steel 11 

Formation  of  beta  iron 11 

Summary • 11 

Another  view  of  the  allotropic  changes 14 

Examination 10 

LESSON  IX  —  THE   THERMAL   CRITICAL  POINTS   OF  IRON  AND   STEEL 

THEIR  EFFECTS 

Changes  at  A3 1 

Dilatation      1 

Electrical  conductivity 1 

Crystallization      2 

Hardness,  ductility,  strength 2 

Dissolving  power  for  carbon 2 

Structural  properties 3 

Other  properties 3 

Changes  at  A2 3 

Dilatation     3 

Magnetic  properties 3 

Crystallization      5 

Hardness,  ductility,  strength 5 

Dissolving  power  for  carbon 5 

Structural  properties 5 

Other  properties 6 

Changes  at  A3.z 6 

Changes  at  AI 6 

Changes  at  A3.2.i 6 

Changes  at  Acm 7 


TABLE  OF  CONTENTS  xi 

PAGE 

Structural  change  at  Ai  and  A3.2.i 7 

Prevailing  conditions  above  and  below  the  critical  range 7 

Properties  of  gamma,  beta,  and  alpha  iron 8 

Examination 8 


LESSON  X  — CAST   STEEL 

Structure  of  cast  eutectoid  steel 1 

Structure  of  cast  hypo-eutectoid  steel      2 

Structure  of  cast  eutectoid  vs.  cast  hypo-eutectoid  steel 3 

Structure  of  cast  hyper-eutectoid  steel 4 

Ingotism 5 

Structure  of  cast  steel  vs.  structure  of  meteorites 6 

Octahedric  crystallization  of  austenite 7 

Experiments 10 

Examination 10 

LESSON  XI  — THE   MECHANICAL   TREATMENT   OF   STEEL 

Hot  working 1 

Finishing  temperatures 3 

Structure  of  hot  worked  eutectoid  steel      4 

Structure  of  hot  worked  hypo-eutectoid  steel 4 

Structure  of  hot  worked  hyper-eutectoid  steel 6 

Sorbite 6 

Hot  working  of  steel  vs.  its  critical  range 7 

Cold  working 8 

Mechanical  refining 9 

Experiments 10 

Examination 10 

LESSON  XII  — THE  ANNEALING   OF   STEEL 

Purpose  of  annealing 1 

Nature  of  the  annealing  operation 1 

Heating  for  annealing 1 

Time  at  annealing  temperature 2 

Cooling  from  annealing  temperature 2 

Rate  of  cooling  vs.  carbon  content 3 

Rate  of  cooling  vs.  size  of  object 3 

Furnace  cooling  from  annealing  temperature 4 

Air  cooling  from  annealing  temperature 4 

Properties  of  sorbite 5 

Influence  of  maximum  temperature      5 

Influence  of  time  at  maximum  temperature (> 

Oil  and  water  quenching  from  annealing  temperature (> 

Double  annealing  treatment 8 

Annealing  eutectoid  steel 10 

Annealing  hypo-eutectoid  steel      

Annealing  hyper-eutectoid  steel 12 

Annealing  steel  castings 13 

Spheroidizing  of  pearlite-cementite 

Varieties  of  pearlite 15 

Graphitizing  of  cementite 

Burnt  steel 17 

Crystalline  growth  of  austenite  above  the  critical  range 

Crystalline  growth  of  ferrite  below  the  critical  range 23 


xii  TABLE  OF  CONTENTS 

PAGE 

Brittleness  of  low  carbon  steel 26 

Experiments 29 

Examination 29 

LESSON  XIII  —  THE   HARDENING   OF   STEEL 

Heating  for  hardening 1 

Cooling  for  hardening 1 

Structural  changes  on  hardening 2 

Austenite 3 

Nature  of  austenite 3 

Occurrence  of  austenite 4 

Etching  of  austenite      6 

Structure  of  austenite 7 

Properties  of  austenite 7 

Martensite 10 

Nature  of  martensite 10 

Occurrence  of  martensite 10 

Etching  of  martensite 10 

Structure  of  martensite 11 

Properties  of  martensite 11 

Troostite 11 

Nature  of  troostite 11 

Occurrence  of  troostite 12 

Properties  of  troostite 13 

Etching  of  troostite 13 

Structure  of  troostite 13 

Sorbite 13 

Troosto-sorbite 15 

Hardenite 15 

Rate  of  cooling  through  critical  range  vs.  structure  of  steel 15 

Are  the  transition  stages  distinct  constituents?      17 

Metarals  and  aggregates 18 

Hardening  eutectoid  steel 18 

Hardening  hyper-eutectoid  steel 18 

Hardening  hypo-eutectoid  steel 18 

Steel  of  maximum  hardening  power 20 

Hardening  large  pieces 20 

Hardening  and  tempering  in  one  operation 20 

Experiments 20 

Etching      21 

Examination 21 

LESSON  XIV  — THE   TEMPERING   OF   HARDENED    STEEL 

Tempering  temperatures 

Tempering  colors 

Time  at  tempering  temperature 1 

Rate  of  cooling  from  tempering  temperature      2 

Hardening  and  tempering  combined 

Explanation  of  the  tempering  of  steel 2 

Tempering  austenitic  steels 3 

Tempering  martensitic  steel 5 

Tempering  t  roost  it  ic  steel 5 

Tempering  troosto-martensitic  steel 5 

Tempering  troosto-sorbitic  steel 5 

Osmondite 5 

Structural  changes  on  slow  cooling,  quick  cooling,  and  reheating 7 


TABLE  OF  CONTENTS  xiii 

PAGE 

Microstructure  of  hardened  and  tempered  steel •  .    .  7 

Carbon  condition  in  tempered  steel      8 

Decrease  of  hardness  on  tempering 9 

Heat  liberated  on  tempering      9 

Experiments 10 

Examination 10 

LESSON  XV  — THEORIES   OF   THE   HARDENING   OF   STEEL 

Retention  theories 1 

Solution  theories 2 

Gamma  iron  theory 2 

Beta  iron  or  allotropic  theory 2 

Alpha  iron  theory _ 4 

Carbon  theories 4 

The  hardening  carbon  theory 4 

The  subcarbide  theory 4 

The  stress  theory 5 

Tempering  and  the  retention  theories 6 

Tempering  and  the  stress  theory 6 

Summary 6 

Examination 7 

LESSON  XVI  — THE   CEMENTATION  AND   CASE   HARDENING   OF   STEEL 

Composition  of  the  iron  and  steel  subjected  to  carburizing 1 

Carburizing  temperature 1 

Time  at  carburizing  temperature 2 

Dist  ribution  of  the  carbon 2 

Carburizing  materials      4 

Mechanism  of  cementation 5 

Cooling  from  carburizing  temperature 5 

Heat  treatment  of  case  hardened  articles 5 

Tempering  case  hardened  steel 6 

Experiments 6 

Examination 6 

LESSON  XVII  —  SPECIAL   STEELS 
GENERAL  CONSIDERATIONS 

Ternary  steels 1 

Influence  of  the  special  element  upon  the  location  of  the  critical  range 3 

Pearlitic  steels 6 

Martensitic  steels 7 

Austenitic  (polyhedric)  steels 7 

Cementitic  (carbide)  steels 8 

Treatments  of  special  steels 8 

Treatment  of  pearlitic  steels      8 

Treatment  of  martensitic  steels 9 

Treatment  of  austenitic  steels 9 

Treatment  of  cementitic  steels 9 

Quaternary  steels 10 

Examination 10 

LESSON  XVIII  —  SPECIAL    STEELS 
CONSTITUTION,  PROPERTIES,  TREATMENT,  AND  USES  OF  MOST  IMPORTANT  TYPES 

Nickel  steel 1 

Manganese  steel 5 

Tungsten  steels 12 


xiv  TABLE  OF  CONTENTS 

PAGE 

Chrome  steels 13 

Vanadium  steels 15 

Silicon  steels 15 

Chrome-nickel  steels 16 

Quaternary  vanadium  steels 17 

Chrome-tungsten  or  high-speed  steels 17 

Experiments     20 

Examination 20 


LESSON  XIX  —  CAST   IRON 

Formation  01  combined  and  graphitic  carbon 1 

Cast  iron  containing  only  graphitic  carbon 1 

Cast  iron  containing  only  combined  carbon .  3 

Cast  iron  containing  both  combined  and  graphitic  carbon 8 

Mottled  cast  iron 10 

Structural  composition  of  cast  iron 10 

Physical  properties  of  cast  iron  vs.  its  structural  composition 11 

Chilled  cast  iron  castings 13 

Examination 13 

LESSON  XX  — IMPURITIES   IN   CAST   IRON 

Silicon  in  cast  iron 1 

Sulphur  in  cast  iron 1 

Manganese  in  cast  iron 2 

Phosphorus  in  cast  iron 2 

Structural  composition  of  phosphoretic  cast  iron 7 

Chemical  vs.  structural  composition 8 

Experiments     9 

Examination 9 


LESSON  XXI  —  MALLEABLE   CAST   IRON 

Graphitizing  of  cementite 1 

Malleable  cast-iron  castings 1 

Original  castings 2 

Annealing  operation 3 

Packing  materials 3 

Annealing  for  malleablizing 4 

Annealing  for  "  white  heart  "  castings 4 

Annealing  for  "  black  heart  "  castings 5 

Gray  cast  iron  vs.  malleable  cast  iron      7 

Experiments     8 

Examination 8 


LESSON  XXII  —  CONSTITUTION   OF   METALLIC   ALLOYS 

Solidification  of  pure  metals 1 

Solidification  of  binary  alloys  the  constituents  of  which  form  solid  solutions 3 

Fusibility  curves  of  binary  alloys  whose  component  metals  are  completely  soluble  in  each  other 

when  solid 5 

Binary  alloys  forming  definite  compounds  and  solid  solutions 8 

Binary  alloys  whose  component  metals  are  insoluble  in  each  other  in  the  solid  state 9 

Binary  alloys  whose  component  metals  are  partially  soluble  in  each  other  when  solid 17 

Examination 21 


TABLE  OF  CONTENTS  xv 
LESSON  XXIII  —  EQUILIBRIUM   DIAGRAM   OF   IRON-CARBON   ALLOYS 

PAGE 

Fusibility  curve  of  iron-carbon  alloys  ..........................  \ 

Structural  composition  of  iron-carbon  alloys  immediately  after  solidification  .........  3 

Iron-graphite  fusibility  curve     .............................  7 

Combined  graphite-cement  ite  diagram    .........................  7 

Graphitizing  of  cementite  ...............................  7 

Structure  of  iron-carbon  alloys  immediately  after  solidification      ..............  10 

Complete  equilibrium  diagram  .............................  12 

Historical  ......................................  16 

Examination     ....................................  21 


LESSON  XXIV  —  THE  PHASE   RULE 

Enunciation  of  the  phase  rule  .............................  1 

Equilibrium  .....................................  1 

Degrees  of  freedom      .................................  2 

Phases    .......................................  3 

Components      ....................................  3 

The  phase  rule  applied  to  alloys   ............................  3 

The  phase  rule  applied  to  pure  metals     .........................  4 

The  phase  rule  applied  to  binary  alloys  .........................  4 

The  phase  rule  applied  to  iron-carbon  alloys  .......................  6 

Examination     ....................................  8 


APPENDIX    I  —  MANIPULATIONS    AND    APPARATUS 

POLISHING  AND  POLISHING  MACHINES     .........................  1 

DEVELOPMENT  OF  THE  STRUCTURES     ..........................  9 

Polishing  in  relief     ................................  9 

Polish-attack     ..................................  9 

Etching     ....................................  10 

Electrolytic  etching     ...............................  11 

Heat  tinting      ..................................  11 

Hot  etching  ...................................  11 

Washing  and  drying    ...............................  11 

Preserving  ....................................  12 

MOUNTING  AND  MOUNTING  DEVICES  ..........................  12 

Plastic  mountings    ................................  12 

Leveling  stages  .................................  14 

METALLURGICAL  MICROSCOPES     ............................  16 

APPENDIX    II  —  NOMENCLATURE    OF    THE    MICROSCOPIC    CONSTITUENTS 

I.    GENERAL  PLAN     ..................................  1 

II.    LIST  OF  MICROSCOPIC  SUBSTANCES   .........................  2 

III.    DEFINITIONS  AND  DESCRIPTIONS    ..........................  4 

Austenite  ....................................  4 

Cementite     ...................................  6 

Martensite   ...................................  7 

Ferrite  .....................................  7 

Osmondite    ...................................  8 

Ferronite  ....................................  9 

Hardenite     ................................... 

Pearlite     ....................................  9 

Graphite  ....................................  1(J 


XVI  TABLE   OF   CONTENTS 

III.  DEFINITIONS  AND  DESCRIPTIONS  —  continued  PAGE 

Troostite 11 

Sorbite 11 

Manganese  sulphide 12 

Ferrous  sulphide 12 

MISCELLANEOUS 12 


INDEX 1-15 


ERRATA 

Lesson  IV,  page  5,  fifth  paragraph,  line  3,  for  "an  iron  carbide  Mn3C"  read  "a  carbide  MnjC." 
Lesson  V,  page  4,  second  paragraph,  line  3,  for  "euctectic"  read  "eutectic." 

Lesson  V,  page  9,  the  first  two  equations  should  read 
"F  =  per  cent  free  ferrite  =  41.18 
P  =  per  cent  pearlite  =  58.82" 

Lesson  V,  page  16,  instead  of  "T  =  1250  P  +  100  (50  -  P)" 
the  third  equation  should  read 

"T  =  1250  P  +  50  (100  -  P)" 

instead  of  "T  =  5000  +  1150  P" 
the  fourth  equation  should  read 
"T  =  5000  +  1200  P" 

instead  of  "T  =  5000  +  1150  80°-120C" 
the  sixth  equation  should  read 

"T  -  5000  +  1200  8°°-120C" 


instead  of  "T 


7 
955,000  -  138,000  C" 


7 
the  seventh  equation  should  read 

_  995,000  -  144,000  C" 

7 

instead  of  "T  =  136,000  -  20,000  C." 
the  eighth  equation  should  read 

"T  =  142,000  -  20,600  C." 

last  line,  instead  of  "111,000"  read  "116,250" 
instead  of  "106,000"  read  "111,100" 

Lesson  V,  page  17,  last  line  of  footnote,  for  "allow  heat  treatment"  read  "allow  for  heat  treatment." 
Lesson  X,  page  6,  eighth  line  from  the  bottom,  for  "peroid"  read  "period." 
Lesson  XV,  page  6,  next  to  last  line,  last  word,  for  "carbon"  read  "iron." 


INTRODUCTION 

THE  INDUSTRIAL  IMPORTANCE  OF  METALLOGRAPHY1 

Twenty  years  ago  the  science  of  metallography  was  practically  unknown  and  it 
is  only  within  the  last  fifteen  years  that  it  has  been  seriously  considered  by  metal 
manufacturers  and  consumers  as  a  valuable  method  of  testing  and  investigating. 
That  so  much  has  been  accomplished  in  so  short  a  time  is  highly  gratifying  to  the 
many  workers,  practical  or  scientific,  who  have  contributed  by  their  efforts  to  the 
progress  of  metallography. 

To  realize  the  practical  importance  of  metallography  it  should  be  borne  in  mind 
that  the  physical  properties  of  metals  and  alloys  —  that  is,  those  properties  to  which 
these  substances  owe  their  exceptional  industrial  importance  —  are  much  more  closely 
related  to  their  proximate  than  to  their  ultimate  composition,  and  that  microscopical 
examination  reveals,  in  part  at  least,  the  proximate  composition  of  metals  and  alloys, 
whereas  chemical  analysis  seldom  does  more  than  reveal  their  ultimate  composition. 

It  will  bear  repeating  that  from  the  knowledge  of  the  proximate  composition  of  a 
certain  industrial  metal  or  alloy  we  are  able  to  infer  its  properties  and,  therefore, 
predict  its  adaptability  with  a  much  greater  degree  of  accuracy  than  if  we  knew  only 
its  ultimate  composition. 

The  analytical  chemist  may  tell  us,  for  instance,  that  a  steel  which  he  has  analyzed 
contains  0.50  per  cent  of  carbon,  without  our  being  able  to  form  any  idea  as  to  its 
properties,  for  such  steel  may  have  a  tenacity  of  some  75,000  Ibs.  per  square  inch  or 
of  some  200,000  Ibs.,  a  ductility  represented  by  an  elongation  of  some  25  per  cent,  or 
practically  no  ductility  at  all;  it  may  be  so  hard  that  it  cannot  be  filed  or  so  soft  as 
to  be  easily  machined,  etc. 

The  metal  microscopist,  on  the  contrary,  on  examining  the  same  steel  will  report 
its  structural,  i.e.  its  proximate,  composition,  informing  us  that  it  contains,  for  in- 
stance, approximately  50  per  cent  of  ferrite  and  50  per  cent  of  pearlite,  and  we  know 
at  once  that  the  steel  is  fairly  soft,  ductile,  and  tenacious;  or  he  may  report  the 
presence  of  100  per  cent  of  martensite,  and  we  know  that  the  steel  is  extremely  hard, 
very  tenacious,  and  deprived  of  ductility. 

Which  of  the  two  reports  is  of  more  immediate  practical  value,  the  chemist's  or 
the  metallographist's?  Surely,  that  of  the  metallographist. 

Nor  is  it  only  in  the  domain  of  metals  that  we  find  such  close  relationship  between 
properties  and  proximate  composition,  for,  on  the  contrary,  it  is  quite  true  of  all 
substances.  How  many  organic  bodies,  for  instance,  have  practically  the  same  ulti- 
mate composition  and  still  are  totally  unlike  in  properties  because  of  their  different 
proximate  composition,  i.e.  different  grouping  and  association  of  their  ultimate  con- 

1  Abstracted  from  a  paper  presented  at  the  Congress  of  Technology  at  the  fiftieth  anniversary 
of  the  granting  of  the  charter  of  the  Massachusetts  Institute  of  Technology. 

1 


W  — 'THE  INDUSTRIAL  IMPORTANCE  OF  METALLOGRAPHY 

stituents.  If  we  were  better  acquainted  with  the  proximate  composition  of  substances 
many  unexplained  facts  would  become  clear  to  us. 

Unfortunately  the  chemist  too  often  is  able  to  give  us  positive  information  in 
regard  to  the  proportion  of  the  ultimate  constituents  only,  his  reference  to  proximate 
analysis  being  of  the  nature  of  speculation.  Ultimate  analysis  has  reached  a  high 
degree  of  perfection  in  regard  to  accuracy  as  well  as  to  speed  of  methods  and  analy- 
tical chemists  have  built  up  a  marvelous  structure  calling  for  the  greatest  admiration, 
their  searching  methods  never  failing  to  lay  bare  the  ultimate  composition  of  sub- 
stances. But  how  much  darkness  still  surrounds  the  proximate  composition  of 
bodies  and  how  great  the  reward  awaiting  the  lifting  of  the  veil! 

The  forceful  and  prophetic  writing  in  1890  of  Prof.  Henry  M.  Howe  naturally 
comes  to  mind.  Speaking  of  the  properties  and  constitution  of  steel,  Professor  Howe 
wrote : 

"If  these  views  be  correct,  then,  no  matter  how  accurate  and  extended  our  knowl- 
edge of  ultimate  composition,  and  how  vast  the  statistics  on  which  our  inferences  are 
based,  if  we  attempt  to  predict  mechanical  properties  from  them  accurately  we  be- 
come metallurgical  Wigginses  .  .  . 

"Ultimate  analysis  never  will,  proximate  analysis  may,  but  by  methods  which 
are  not  yet  even  guessed  at,  and  in  the  face  of  fearful  obstacles. 

"How  often  do  we  look  for  the  coming  of  the  master  mind  which  can  decipher  our 
undecipherable  results  and  solve  our  insoluble  equations,  while  if  we  will  but  rub  our 
own  dull  eyes  and  glance  from  the  petty  details  of  our  phenomena  to  their  great  out- 
lines their  meaning  stands  forth  unmistakably;  they  tell  us  that  we  have  followed 
false  clues  and  paths  which  lead  but  to  terminal  morasses.  In  vain  we  flounder  in 
the  sloughs  and  quagmires  at  the  foot  of  the  rugged  mountain  of  knowledge  seeking 
a  royal  road  to  its  summit.  If  we  are  to  climb,  it  must  be  by  the  precipitous  paths 
of  proximate  analysis,  and  the  sooner  we  are  armed  and  shod  for  the  ascent,  the  sooner 
we  devise  weapons  for  this  arduous  task,  the  better. 

"By  what  methods  ultimate  composition  is  to  be  determined  is  for  the  chemist 
rather  than  the  metallurgist  to  discover.  But,  if  we  may  take  a  leaf  from  lithology, 
if  we  can  sufficiently  comminute  our  metal  (ay,  there's  the  rub!)  by  observing  dif- 
ferences in  specific  gravity  (as  in  ore  dressing),  in  rate  of  solubility  under  rigidly  fixed 
conditions,  in  degree  of  attraction  by  the  magnet,  in  cleavage,  luster,  and  crystalline 
form  under  the  microscope,  in  readiness  of  oxidation  by  mixtures  of  gases  in  rigidly 
fixed  proportions,  we  may  learn  much. 

"Will  the  game  be  worth  the  candle?  Given  the  proximate  composition,  will  not 
the  mechanical  properties  of  the  metal  be  so  greatly  influenced  by  slight  and  unde- 
terminable changes  in  the  crystalline  form,  size,  and  arrangement  of  the  component 
minerals,  so  dependent  on  trifling  variations  in  manufacture  as  to  be  still  only  roughly 
deducible?" 

The  above  was  written  before  the  days  of  metallography,  or  at  least  when  metal- 
lography had  barely  appeared  in  the  metallurgical  sky  and  when  no  one  yet  had  fan- 
cied what  would  be  the  brilliant  career  of  the  newcomer.  Metallography  has  done 
much  to  supply  the  need  so  vividly  and  timely  depicted  by  Professor  Howe,  precisely 
because  by  lifting  a  corner  of  the  veil  hiding  from  our  view  the  proximate  composi- 
tion of  metals  and  alloys  it  has  thrown  a  flood  of  light  upon  the  real  constitution  of 
these  important  products.  Has  the  game  been  worth  the  candle?  Will  any  one 
hesitate  to  answer  in  the  affirmative  Professor  Howe's  question? 


INTRODUCTION  — THE  INDUSTRIAL  IMPORTANCE  OF  METALLOGRAPHY      3 

Professor  Howe  with  his  usual  acumen  was  conscious  of  the  fact  that  proximate 
analysis,  while  likely  to  reveal  a  great  deal  more  of  the  constitution  of  metals  than 
ultimate  analysis  ever  could,  might  still  leave  us  in  such  ignorance  of  their  physical 
structure  as  to  throw  but  little  additional  light  upon  the  subject.  His  fear  was  cer- 
tainly well  founded  and  surely  if  the  proximate  composition  had  been  obtained  by 
chemical  analysis  it  would  indeed  have  told  us  little  of  the  structure  or  anatomy  of 
the  metals.  In  the  domain  of  proximate  composition  chemistry  cannot  do  more  for 
the  metallurgist  than  it  does  for  the  physician. 

Invaluable  information  chemistry  does  give,  without  which  both  the  physician  and 
the  metallurgist  would  be  in  utter  darkness,  but  it  throws  little  or  no  light  upon  the 
anatomy  of  living  or  inanimate  matter.  Its  very  methods  which  call  for  the  destruc- 
tion of  the  physical  structure  of  matter  show  how  incapable  it  is  to  render  assistance 
in  this,  our  great  need. 

The  parallel  drawn  here  between  metals  and  living  matter  is  not  fantastic.  It 
has  been  aptly  made  by  Osmond,  who  said  rightly  that  modern  science  was  treating 
the  industrial  metal  like  a  living  organism  and  that  we  were  led  to  study  its  anatomy, 
i.e.  its  physical  and  chemical  constitution;  its  biology,  i.e.  the  influence  exerted  upon 
its  constitution  by  the  various  treatments,  thermal  and  mechanical,  to  which  the 
metal  is  lawfully  subjected;  and  its  pathology,  i.e.  the  action  of  impurities  and 
defective  treatments  upon  its  normal  constitution. 

Fortunately  metallography  does  more  than  reveal  the  proximate  composition  of 
metals.  It  is  a  true  dissecting  method  which  lays  bare  their  anatomy  —  that  is,  the 
physical  grouping  of  the  proximate  constituents,  their  distribution,  relative  dimen- 
sions, etc.,  all  of  which  necessarily  affect  the  properties.  For  two  pieces  of  steel,  for 
instance,  might  have  exactly  the  same  proximate  composition  —  that  is,  might  con- 
tain, let  us  say,  the  same  proportion  of  pearlite  and  ferrite  and  still  differ  quite  a 
little  as  to  strength,  ductility,  etc.,  and  that  because  of  a  different  structural  arrange- 
ment of  the  two  proximate  constituents;  in  other  words,  because  of  unlike  anatomy. 

It  is  not  to  be  supposed  that  the  path  trodden  during  the  last  score  of  years  was 
at  all  times  smooth  and  free  from  obstacles.  Indeed,  the  truth  of  the  proverb  that 
there  is  no  royal  road  to  knowledge  was  constantly  and  forcibly  impressed  on  the 
mind  of  those  engaged  in  the  arduous  task  of  lifting  metallography  to  a  higher  level. 

Its  short  history  resembles  the  history  of  the  development  of  all  sciences.  At  the 
outset  a  mist  so  thick  surrounds  the  goal  that  only  the  most  courageous  and  better 
equipped  attempt  to  pierce  it  and  perchance  they  may  be  rewarded  by  a  gleam  of 
light.  This  gives  courage  to  others  and  the  new  recruits  add  strength  to  the  besieg- 
ing party.  Then  follows  the  well-known  attacking  methods  of  scientific  tactics  and 
strategy,  and  after  many  defeats  and  now  and  then  a  victorious  battle  the  goal  is  in 
sight,  but  only  in  sight  and  never  to  be  actually  reached,  for  in  our  way  stands  the 
great  universal  mystery  of  nature :  what  is  matter?  what  is  life? 

Nevertheless  there  is  reward  enough  for  the  scientist  in  the  feeling  that  he  has 
approached  the  goal,  that  he  has  secured  a  better  point  of  vantage  from  which  to 
contemplate  it.  The  game  was  worth  the  candle.  And  if  scientific  workers  must 
necessarily  fail  in  their  efforts  to  arrive  at  the  true  definition  of  matter,  whatever  be 
the  field  of  their  labor,  they  at  least  learn  a  great  deal  concernirig  the  ways  of  matter, 
and  it  is  with  the  ways  of  matter  that  the  material  world  is  chiefly  concerned.  Hence 
the  usefulness  of  scientific  investigation,  hence  the  usefulness  of  metallography. 

Like  any  other  science  with  any  claim  to  commercial  recognition,  metallography 


4      INTRODUCTION  — THE  INDUSTRIAL  IMPORTANCE  OF  METALLOGRAPHY 

has  had  first  to  withstand  the  attack  and  later  to  overcome  the  ill-will  and  reluctance 
of  the  so-called  "practical  man"  with  a  decided  contempt  for  anything  scientific. 
He  represents  the  industrial  philistine  clumsily  standing  in  the  way  of  scientific  ap- 
plications to  industrial  operations.  Fortunately,  while  his  interference  may  retard 
progress,  it  cannot  prevent  it.  Had  he  had  his  own  way  neither  the  testing  machine, 
nor  the  chemical  laboratory,  nor  the  metallographical  laboratory,  nor  the  pyrometer 
would  ever  have  been  introduced  in  iron  and  steel  works. 

Speaking  in  1904  of  the  practical  value  of  metallography  in  iron  and  steel  making, 
the  author  wrote  the  following,  which  it  may  not  be  out  of  place  to  reproduce  here : 
"History,  however,  must  repeat  itself,  and  the  evolution  of  the  metallographist  bids 
fair  to  be  an  exact  duplicate  of  the  evolution  of  the  iron  chemist ;  the  same  landmarks 
indicate  his  course;  distrust,  reluctant  acceptance,  unreasonable  and  foolish  expecta- 
tion from  his  work,  disappointment  because  these  expectations  were  not  fulfilled  and 
finally  the  finding  of  his  proper  sphere  and  recognition  of  his  worth.  The  metal- 
lographist has  passed  through  the  first  three  stages  of  this  evolution,  is  emerging 
from  the  fourth,  and  entering  into  the  last.  For  so  young  a  candidate  to  recognition 
in  iron  and  steel  making  this  record  is  on  the  whole  very  creditable." 

We  may  say  to-day  that  he  has  definitely  entered  the  last  stage  and  that  the  ad- 
verse criticisms  still  heard  from  time  to  time,  generally  from  the  pen  or  mouth  of 
ignorant  persons,  are  like  the  desultory  firing  of  a  defeated  and  retreating  enemy. 

In  the  United  States  alone  the  microscope  is  in  daily  use  for  the  examination  of 
metals  and  alloys  in  more  than  two  hundred  laboratories  of  large  industrial  firms, 
while  metallography  is  taught  in  practically  every  scientific  or  technical  school. 

A.  S. 

HARVARD  UNIVERSITY, 
February,  1912. 


APPARATUS  FOR  THE  METALLO GRAPHIC  LABORATORY.1 

Only  those  apparatus  which  the  author  has  found  most  satisfactory  are  here  men- 
tioned —  other  instruments  will  be  found  described  in  an  Appendix. 

THE  MICROSCOPE 

While  any  good  microscope  of  the  ordinary  type,  substantially  built  and  provided 
with  a  satisfactory  fine  adjustment,  may  be  used  with  a  certain  degree  of  success  for 
the  examination  of  metals  and  alloys,  those  who  are  restricted  to  its  use  will  soon 
find  themselves  seriously  handicapped  in  several  directions  and  unable  to  obtain  the 
desired  results.  The  following  considerations  will  make  this  clear. 

The  Stage.  —  Ordinary  microscope  stands  being  constructed  for  the  examination 
of  objects  by  transmitted  light,  i.e.  by  light  proceeding  from  below  the  stage  and 
passing  through  the  object  on  its  way  to  the  eye,  are  provided  with  fixed  stages.  This, 
however,  is  a  serious  objection  when  the  instrument  is  applied  to  the  examination  of 
metals  and  other  opaque  objects  which  must  necessarily  be  illuminated  by  light  di- 
rected upon  them  from  above  the  stage,  and  which  therefore  require  the  use  of  an 
"  illuminator  "  attached  to  the  objective  and  consequently  moving  with  it.  It  will 
be  readily  understood  that  it  is  of  considerable  importance  that  the  position  of  this 
illuminator,  and  therefore  of  the  objective  to  which  it  is  attached,  be  kept  constant, 
once  the  necessary  adjustments  are  effected,  since  any  change  in  its  position  would 
require  a  readjustment  of  the  source  of  light,  the  condensing  lenses,  diaphragm,  etc. 
To  that  effect  the  stage  should  be  provided  with  a  rack  and  pinion  motion  by  means 
of  which  the  coarse  focusing  at  least  may  be  done  (Fig.  1). 

This  rack  and  pinion  motion  of  the  stage,  moreover,  permits  of  a  much  greater 
working  distance,  allowing  plenty  of  room  for  the  insertion  of  the  illuminator  and 
nose-piece,  the  use  of  specimen  holders,  and  the  examination  of  bulky  specimens  with 
low-power  objectives.  In  the  microscope  illustrated  in  Figure  1,  the  working  distance 
measures  over  5  inches  as  against  4  inches  or  less  in  ordinary  stands. 

By  not  departing  more  than  necessary  from  the  usual  construction  of  microscopes, 
none  of  the  essential  features  required  for  the  examination  of  transparent  prepara- 
tions need  be  sacrificed,  and  the  full  efficiency  of  the  microscope  is  retained  for  such 
examination,  sub-stage  condensers,  polarizing  prisms,  etc.,  being  readily  attached 
when  needed.  The  possibility  of  applying  his  instrument  to  all  kinds  of  microscopical 
work  with  equally  satisfactory  results  should  appeal  strongly  to  the  metallographist, 
for  there  is  no  laboratory  where,  occasionally  at  least,  examination  of  transparent 
objects  is  not  desirable  or  even  imperative. 

1  Abstracted  in  part  from  papers  by  the  author  on  "Apparatus  for  the  Microscopical  Examina- 
tion of  Metals,"  American  Society  for  Testing  Materials,  Vol.  X,  1910;  and  "The  Universal  Metal- 
loscope,"  American  Institute  of  Mining  Engineers,  June,  1911. 

1 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


Fig.  1.  —  Metallurgical  microscope,  eye-piece,  vertical  illuminator,  objective, 
magnetic  specimen  holder,  and  mechanical  stage. 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY          3 

A  less  expensive  but  satisfactory  microscope  is  shown  in  Figures  2  and  3.  The 
latter  illustration  includes  an  auxiliary  tube  inserted  between  the  objective  and  illu- 
minator for  the  examination  of  large  samples  which  may  be  placed  on  the  base  of  the 
microscope  or  supported  in  some  other  suitable  way  below  the  stage. 

(a)  Plain  Stages.  —  While  a  mechanical  stage  adds  greatly  to  the  convenience  of 
the  manipulations,  a  plain  stage  may  be  used  with  satisfactory  results.    It  should  be 
provided  with  strong  clips  to  hold  in  place  the  specimen  holders  soon  to  be  described, 
and  should  preferably  be  circular  and  revolving  (Fig.  4).    When  provided  with  cen- 
tering screws  like  the  stage  of  the  stand  illustrated  in  Figure  1,  the  object  may  be  moved 
gently  while  under  examination,  a  very  desirable  feature  especially  when  using  high- 
power  objectives,  in  which  case  the  moving  of  the  object  entirely  by  hand  is  very  jerky. 
In  order  to  derive  the  full  benefit  of  the  use  of  the  magnetic  holder  described  later, 
the  central  opening  of  the  stage  should  not  be  less  than  1J4  inches  in  diameter. 

(b)  Mechanical  Stages.  —  The  great  superiority  of  a  mechanical  stage  permitting, 
as  it  does,  a  systematic  examination  of  the  object  over  its  entire  surface,  need  not  be 
insisted  upon.    In  connection  with  the  magnetic  holder  it  makes  it  possible,  moreover, 
to  examine  repeatedly  and  at  any  time  the  same  spot  of  any  specimen,  as  will  soon  be 
explained.    The  mechanical  stage  illustrated  in  Figure  5  has  been  especially  designed 
to  fit  the  metallurgical  microscope  (Fig.  1),  and  is  very  readily  substituted  for  the  plain 
stage.    The  central  opening  measures  1^  inches  in  diameter,  permitting  the  convenient 
use  of  the  magnetic  holder. 

Objectives.  —  Ordinary  achromatic  objectives  give  satisfactory  results.  They 
should,  however,  be  corrected  for  uncovered  objects,  as  the  placing  of  cover  glasses 
over  bright  metallic  surfaces  is  accompanied  by  light  reflection  causing  loss  of  clearness 
and  definition. 

Some  believe  that  the  objectives  should  be  provided  with  short  mounts  so  as  to 
bring  the  reflector  of  the  vertical  illuminator  as  near  the  back  lens  of  the  objective  as 
possible,  and  thus,  in  their  opinion,  materially  decreasing  the  amount  of  glare  caused 
by  the  reflection  of  light  by  the  lenses  of  the  objectives.  Three  objectives,  one  of  low, 
one  of  medium,  and  one  of  high  power,  will  generally  suffice  for  metallographic  work. 
The  following  focal  lengths  are  recommended:  32-mm.  or  1^-in.,  16-mm.  or  %-m.., 
and  4-mm.  or  Y§-\\\.  These  objectives  are  shown,  in  short  mounts,  in  Figure  6.  The 
32-mm.  objective  is  provided  with  a  society  screw  at  its  lower  end  in  order  that  the 
vertical  illuminator  may  be  inserted  between  the  objective  and  the  object,  this  being 
desirable  with  very  low-power  lenses.  In  case  a  higher  power  is  needed,  a  2-mm.  or 
lV~m-  oil  immersion  objective  will  be  found  very  satisfactory.  When  a  very  low- 
power  lens  is  required,  as  for  instance  in  the  examination  of  fractures  or  of  very  coarse 
structures,  a  48-mm.  or  2-in.  objective  will  give  good  results.  It  is  suggested  that  it 
be  provided  at  its  lower  end  with  a  society  screw  to  permit  the  attachment  of  the  ver- 
tical illuminator,  which  in  the  case  of  such  low-power  lenses  should  be  placed 
between  the  object  and  the  objective,  as  explained  later. 

Eye-Pieces.  —  With  achromatic  objectives  ordinary  Huygenian  eye-pieces  are 
used.  Two  eye-pieces,  respectively  of  1-in.  and  2-in.  focal  length,  will  generally  cover 
the  range  of  magnification  needed. 

For  the  taking  of  photomicrographs,  projection  eye-pieces  are  said  to  possess 
some  superiority,  especially  when  high-power  objectives  are  used,  as  they  then  yield 
flatter  and  more  sharply  defined  images.  The  Zeiss  projection  eye-piece  No.  2  is  very 
satisfactory. 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


Fig.  2.  —  Student  microscope  (.6  actual  size). 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


Fig.  3.  —  Student  microscope  fitted  with  auxiliary  tube. 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


r 


Fig-  4.  —  Plain  revolving  stage,  magnetic  specimen 
holder,  and  specimen. 


Fig.  5.  —  Mechanical  stage  to  fit  metallurgical  microscope, 
magnetic  specimen  holder,  and  specimen. 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


Iris  Diaphragms.  —  Iris  diaphragms  are  sometimes  inserted  between  the  objec- 
tives and  the  illuminator  so  as  to  control  the  size  of  the  pencil  of  light  proceeding  from 
the  object,  with  a  view  of  securing  sharper  definition.  Their  use  in  that  position, 
however,  is  of  doubtful  value,  as  it  may  cause  some  distortion  of  the  image.  It  seems 
preferable  to  place  the  iris  diaphragm  between  the  source  of  light  and  the  illuminator, 
thus  regulating  the  amount  of  light  entering  the  latter.  When  placed  between  the 
objective  and  the  illuminator  it  increases,  moreover,  their  distance  apart,  which  we 
have  seen  to  be  objectionable.  If  a  diaphragm  must  be  attached  to  the  microscope, 
it  is  better  to  place  it  between  the  tube  nose  and  the  illuminator.  When  using  low- 


32  mm.  1(5  mm. 

Fig.  6.  —  Short  mounted  achromatic  objectives. 


4  mm. 


power  lenses  it  might  also  be  screwed  to  the  lower  end  of  the  objective,  thus  control- 
ling the  light  returned  by  the  object  before  entering  the  objective. 

Specimen  Holders.  —  In  order  to  examine  a  piece  of  metal  under  the  microscope, 
it  is  of  course  necessary  that  the  polished  and  otherwise  prepared  surface  be  held  in 
a  plane  accurately  perpendicular  to  the  optical  axis  of  the  instrument.  This  may  be 
accomplished  by  so  shaping  the  sample  that  it  will  have  two  sides  exactly  parallel, 
and  preparing  one  of  them  for  microscopical  examination.  This  operation,  however, 
is  at  best  tedious  and  laborious,  and  metallographists  have  endeavored  to  replace  it 
by  the  use  of  more  or  less  ingenious  devices  for  holding  the  specimens  in  the  proper 


Fig.  7.  —  Specimen  holder. 

position.  Some  embed  their  samples  in  wax  or  in  some  other  plastic  material,  while 
others  have  recourse  to  stages  provided  with  special  leveling  devices. 

The  simple  holder  shown  in  Figure  7  gives  better  satisfaction,  requiring  no  mount- 
ing whatever  of  the  samples.  The  specimen,  no  matter  how  irregular  in  shape,  is 
held  firmly  in  place  by  a  rubber  band  and  the  holder  placed  on  the  stage  like  an 
ordinary  slide.  If  the  correction  of  the  objective  demands  it,  a  cover  glass  may  be 
inserted  between  the  sample  and  the  holder.  It  will  be  apparent  that  the  required 
manipulations  are  very  simple  and  quickly  performed. 

In  the  case  of  specimens  smaller  than  the  opening  of  the  holder,  however,  the  use 
of  a  cover  glass  is  necessary  to  hold  them  in  place.  This  is  objectionable,  at  least 


8          APPARATUS  FOR  THE  METALLOGRAPHIO  LABORATORY 

when  using  high-power  objectives,  which  should  be  corrected  for  uncovered  objects. 
To  overcome  this  difficulty,  a  little  templet  may  be  used  having  a  triangular  opening 
and  inserted  between  the  specimen  and  the  holder  (Fig.  8).  This  templet  is  made 
very  thin  so  as  to  permit  the  use  of  high-power  objectives,  which  must  be  brought 
very  close  indeed  to  the  object.  It  will  also  be  noticed  that  one  side  of  the  upper  part 
of  the  holder  has  been  removed,  exposing  to  .view  a  larger  portion  of  the  sample  and 
permitting  a  more  ready  approach  of  high-power  objectives.  Large  samples  are,  of 
course,  placed  in  the  holder  without  any  templet. 

A  still  simpler  and  more  effective  device  can  be  used  to  hold  in  place  samples  of 
iron  and  steel  and  other  magnetic  substances.     The  device  consists  of  a  V-shaped 


(a) 


(6) 


Fig.  8.  —  (a)  Specimen  holder  and  large  sample. 

(6)  Specimen  holder,  templet,  and  small 
sample. 


permanent  magnet  of  special  steel  about  1  inch  wide  and  2J^  inches  long  (Fig.  9). 
This  little  magnet  is  placed  on  the  stage  of  the  microscope  like  an  ordinary  glass  slide 
(Figs.  4  and  5)  and  tne  sample  to  be  examined  suspended  to  it  from  below,  being  held 
in  place  by  the  attraction  of  both  poles.  Small  samples  are  suspended  near  the  small 
end  of  the  V-shaped  opening,  while  larger  ones  are  placed  nearer  the  wider  end  of  the 
opening.  This  holder,  therefore,  is  universal  in  its  application  within  the  limits  of 
samples  of  suitable  size  for  microscopical  examination.  If  the  opening  of  the  stage 
be  sufficiently  large,  say  \l/±  inches  or  more  in  diameter,  the  magnet  may  be  kept  per- 
manently on  the  stage,  as  the  samples  may  then  be  readily  removed  or  attached  to 
the  magnet  with  the  fingers  from  below  the  stage.  This  adds  so  much  to  the  conve- 
nience of  this  little  device  that  it  is  strongly  urged,  in  case  the  central  aperture  of  the 
stage  is  too  small,  to  have  it  suitably  enlarged.  The  magnet  is  kept  in  place,  like  any 
glass  slide,  by  the  clips  of  the  microscope  and,  also  like  any  glass  slide,  may  be  moved 
about  for  the  inspection  of  the  different  parts  of  the  specimen.  The  side  of  the  magnet 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


9 


resting  on  the  stage  having  been  ground  perfectly  flat,  it  will  be  evident  that  the  sur- 
face of  the  sample  under  examination  will  always  be  accurately  in  the  proper  position, 
permitting  the  use  of  high-power  objectives  without  fear  of  difficulty  arising  from  ever 
so  slight  an  inclination  of  the  sample. 

When  used  in  connection  with  a  mechanical  stage  (Fig.  5)  the  convenience  of  this 
little  holder  becomes  still  more  apparent  and  its  usefulness  is  further  increased.  It 
then  affords,  moreover,  a  ready  means  for  the  repeated  examination  of  the  same  spot 
of  any  sample  at  any  time.  To  that  effect  the  holder  is  laid  upon  the  prepared  surface 
and  two  scratches  made  by  drawing  a  needle  across  the  specimen  along  the  sides  of 


Fig.  9. 


(a) 


(6) 


\"  /  v, 

-  (a)  Magnetic  specimen  holder 
(b)  Scratched  specimen. 


the  V-shaped  opening,  as  shown  in  Figure  9.  When  it  is  desired  to  examine  the  sam- 
ple, the  latter  is  suspended  to  the  magnet  so  that  the  needle  markings  coincide  closely 
with  the  sides  of  the  magnet  opening,  in  this  way  securing  a  permanent  position  for 
the  sample.  The  position  of  the  magnet  itself  is  controlled,  in  the  usual  way,  by  means 
of  the  graduating  devices  of  the  mechanical  stage. 

Finally,  by  placing  the  sample  below  the  stage  and  bringing  the  prepared  surface 
on  a  level  with  the  stage,  considerably  greater  working  distance  is  secured,  a  gain 
which  has  its  importance. 

The  only  limitation  of  this  holder  is  due  to  the  fact  that  with  very  small  speci- 


Fig.  10.  —  (a)  Magnetic  holder. 

(b)  Steel  templet. 

(c)  Magnetic  holder,  templet,  and  sample. 


mens  it  is  impossible  to  use  high-power  objectives  (^  inch  or  less  in  focal  length), 
because  the  mounting  of  the  objective  comes  in  contact  with  the  sides  of  the  magnet 
and  prevents  the  focusing  of  the  object.  For  the  examination  by  high-power  objec- 
tives, the  use  of  a  very  thin  steel  templet  (not  over  0.01  inch  thick)  of  the  same  dimen- 
sions as  the  magnetic  holder,  but  with  a  V-shaped  opening  considerably  narrower 
(Fig.  10)  is  recommended.  This  templet  is  placed  over  the  magnetic  holder  so  as  to 
exactly  cover  it,  thereby  becoming  magnetized.  The  small  iron  and  steel  samples 
are  suspended  to  this  thin  steel  plate  in  the  usual  way  and  may  then  be  examined 
with  the  highest  power  objectives. 


10         APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 

Universal  Metalloscope.  —  The  instrument  shown  in  Figures  11  to  15  was  devised 
especially  for  the  ready  examination  of  large  iron  and  steel  samples,  but  it  will  be  ap- 
parent that  it  can  be  used  with  equally  satisfactory  results  for  small  samples  both 
opaque  and  transparent. 

The  microsope  stand  proper  consists  of  a  microscope-tube,  provided  with  both 
coarse  and  fine  adjustments,  and  with  a  draw-tube,  rigidly  mounted  on  a  bar  supported 
at  both  ends  on  substantial  and  firm  cast-iron  legs.1  The  height  between  the  table 
and  the  under  side  of  the  supporting  bar  is  5  inches  and  the  distance  between  the  sup- 
porting legs  12  inches. 

This  arrangement  affords  free  space  below  the  objective  for  the  examination  of 


Fig.  11.  —  Universal  metalloscope:  stand,  eye-piece,  vertical  illuminator,  objec- 
tive, electromagnetic  stage,  and  rail  section. 

large  specimens  of  metals,  such  as  full  rail  sections,  without  detracting  in  the  least 
from  the  value  of  the  instrument  when  applied  to  the  examination  of  the  usual  small 
specimen,  as  explained  later.  Many  metal  microscopists  frequently  have  to  examine 
bulky  specimens,  and  this  is  altogether  impossible  with  the  ordinary  microscopes  as 
well  as  with  the  special  metallurgical  microscopes  which  have  been  designed  and  de- 
scribed from  time  to  time. 

Recourse  must  be  had  to  all  sorts  of  makeshifts  for  the  proper  support  of  large 
specimens,  or,  more  often,  the  microscopist  gives  up  the  attempt  altogether,  or  else 
resigns  himself  to  the  cutting  of  the  bulky  samples  into  small  pieces  to  be  laboriously 
polished  and  separately  examined. 

It  is  believed  that  an  instrument  permitting  the  examination  of  large  as  well  as 

1  In  a  more  recent  model  the  supporting  bar  is  mounted  on  three  legs,  permitting  the  ready  lev- 
eling of  the  instrument. 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


11 


of  small  specimens  with  equal  ease  and  accuracy  will  be  welcomed  by  metallographists, 
and  that  it  will  lead  to  more  frequent  examinations  of  full  sections  of  metal  imple- 
ments, a  departure  which  should  bring  fruitful  results. 

Electromagnetic  Stage.  —  The  perplexing  question  of  the  proper  support,  for  mi- 
croscopical examination,  of  iron  and  steel  specimens  of  all  sizes  and  shapes  has  been 
effectively  solved  by  the  use  of  the  electromagnetic  stage  illustrated  in  Figure  11. 
This  stage  consists  of  a  steel  plate  7  by  14  inches  having  a  V-shaped  opening,  and  con- 
verted into  a  powerful  electromagnet  by  means  of  two  bobbins  with  solenoids  sur- 
rounding the  arms  of  the  steel  plate,  as  clearly  shown  in  the  illustration.  Electrical 
connection  is  readily  made  with  any  suitable  current,  and  the  use  of  an  incandescent 
lamp  in  series  provides  in  a  simple  way  the  necessary  outside  resistance  to  prevent 
heating  of  the  solenoids.  Large  specimens  of  iron  and  steel,  such  as  rail  sections, 
A,  Figure  12,  are  firmly  held  in  an  accurate  position  by  the  attraction  of  the  mag- 
netic stage,  the  extremities  of  the  flange  only  and  a  narrow  space  on  each  side  of  the 


"ih 


IA1 


ABC 
Fig.  12.  —  (A)  Electromagnetic  stage  and  rail  section.     (B)  Electromag- 
netic stage,  templet,  and  medium-size  specimen.     (C)  Electromag- 
netic stage,  two  templets,  and  small  specimen. 

head  being  hidden  from  view.  The  size  and  shape  of  the  stage-opening  make  pos- 
sible the  ready  support  of  specimens  measuring  from  2  to  6  inches  in  their  greatest 
dimension. 

Templets  for  the  Examination  of  Small  Specimens.  —  For  the  examination  of  iron 
and  steel  samples  from  2  inches  in  length  down  to  the  smallest  dimensions,  a  steel 
templet,  also  with  a  V-shaped  opening,  is  placed  on  the  stage,  shown  at  B,  Figure  12. 
This  templet  through  its  contact  with  the  stage  becomes  strongly  magnetized  and  the 
specimens  to  be  examined  are  suspended  to  it. 

For  the  examination  of  very  small  specimens  with  high-power  lenses  the  thickness 
of  this  templet  would  prevent  the  necessary  close  approach  of  the  objective.  To 
make  this  approach  possible  a  very  thin  steel  templet  (not  exceeding  0.01  inch  thick) 
is  used,  shown  at  C,  Figure  12,  which  makes  possible  actual  contact  between  a  high- 
power  objective  and  the  smallest  specimen. 

Support  of  Non-Magnetic  Specimens.  —  For  the  support  of  non-magnetic  speci- 
mens, such  as  non-ferrous  metals,  rocks,  cement,  etc.,  a  very  simple  device  is  provided, 
consisting  of  two  crossbars  and  rubber  bands,  which  is  readily  attached  to  the  stage 
and  by  means  of  which  the  non-magnetic  specimens,  as  well  as  the  templets  when 
needed,  are  firmly  held  in  place  regardless  of  their  size  or  shape. 


12 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


Leveling- Devices  of  Stand  and  Stage.  —  It  is,  of  course,  essential,  especially  when 
using  high-power  objectives,  that  the  optical  axis  of  the  microscope  be  accurately 
perpendicular  to  the  surface  under  examination.  To  secure  this  result  both  the  stand 
and  the  stage  are  provided  with  leveling-screws,  as  shown  in  Figure  11.  For  leveling 
the  stage  a  small  spirit-level  may  be  placed  upon  it,  or  better,  upon  the  sample  under 
examination,  and  the  necessary  adjustment  quickly  made.  For  leveling  the  micro- 
scope stand  the  eye-piece  should  be  removed,  the  small  level  placed  on  top  of  the  tube, 
and  the  leveling-screws  adjusted.  By  placing  the  instruments  on  a  table  or  desk 
having  a  smooth  and  flat  top,  it  is  evident  that,  barring  accidents,  the  stand  and 
stage  will  remain  indefinitely  accurately  leveled. 

Motion  of  the  Stage.  —  In  order  to  examine  the  entire  surface  of  a  large  specimen 
it  is  necessary  to  bring  in  turn  within  the  field  of  the  microscope  the  different  portions 


Fig.  13.  —  Back  leg  of  electromagnetic  stage  and  sliding  plate. 

of  the  specimen,  and  this  necessitates  the  moving  of  the  stage  in  various  directions. 
The  weight  of  the  stage,  however,  would  create  considerable  friction  between  the  legs 
and  the  supporting  table,  making  the  sliding  motion  jerky  and  otherwise  unsteady. 
To  overcome  this  difficulty  the  back  leg  of  the  stage  is  provided  with  a  small  wheel 
running  in  a  groove  cut  in  a  small  brass  plate  fastened  to  the  table  or  desk,  shown  in 
Figure  13.  The  mounting  of  the  wheel  is  provided  with  a  pivot  fitting  snugly  into  a 
hole  in  the  leg.  This  construction  makes  possible  the  ready  back-and-forth  motion 
of  the  stage,  as  well  as  its  free  circular  displacement  around  the  axis  of  the  back  leg 
thus  permitting  to  bring  quickly  any  desired  portion  of  the  object  under  the  objec- 
tive. As  the  bulk  of  the  weight  is  supported  by  the  back  leg,  the  arrangement  makes 
possible  a  very  steady  and  smooth  motion  of  the  stage. 

Mechanical  Stage.  —  The  use  of  a  mechanical  stage  is  often  highly  desirable.  This 
is  taken  care  of  in  the  present  instrument  in  two  different  ways:  (1)  by  the  use  of  a 
mechanical  stage  suitably  attached  to  the  electromagnetic  stage,  and  (2)  by  the  use 
of  a  mechanical  stage  independently  mounted  on  a  separate  base  of  the  usual  horseshoe 
pattern. 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


13 


The  first  method  is  illustrated  in  Figure  14.  A  mechanical  stage  of  usual  construc- 
tion is  screwed  on  a  brass  plate  provided  with  two  small  pins  fitting  two  correspond- 
ing holes  in  the  magnetic  stage,  thus  securing  a  firm  and  constant  position  for  the 
mechanical  stage.  When  using  a  mechanical  stage,  however,  a  rigid  and  constant 
position  should  also  be  secured  between  it  and  the  microscope  stand.  To  that  effect 
a  brass  plate  is  provided,  with  recesses  to  receive  the  back  legs  of  the  stand  as  well 
as  the  front  legs  of  the  stage,  shown  in  Figure  14.  It  is  then  possible  at  any  time  to 
place  the  microscope  stand  and  the  stage  in  exactly  the  same  relative  positions. 

The  second  method  consists  in  the  use  of  a  mechanical  stage  separately  mounted 
on  an  ordinary  horseshoe  base,  shown  in  Figure  15.  To  secure  a  constant  relative 


Fig.  14.  —  Universal  metalloscope:  electromagnetic  stage  with  mechanical 
stage,  magnetic  specimen  holder,  small  specimen,  and  base-plate. 

position  between  stand  and  stage,  the  foot  of  the  latter  fits  into  recesses  provided  for 
that  purpose  in  the  base-plate. 

The  use  of  this  independently  mounted  mechanical  stage  offers  the  additional 
advantage  resulting  from  the  vertical  up-and-down  racking  of  the  stage,  rendering 
unnecessary  any  vertical  adjustment  of  the  light  and  condenser,  as  well  understood 
by  metallographists. 

Examination  of  Transparent  Objects.  —  To  adapt  the  universal  metalloscope  to 
the  examination  of  transparent  objects,  thereby  converting  it  into  an  ordinary  micro- 
scope, or,  if  desired,  into  a  petrographical  microscope,  a  separate  stage  on  horseshoe 
base  should  be  used,  as  shown  in  Figure  15,  when  the  necessary  Abbe  condenser, 
analyzer,  polarizer,  etc.,  can  readily  be  attached.  The  instrument  is  then  in  no 
way  inferior  to  high-class  microscopes  for  examination  by  transmitted  or  polarized 
light. 


14 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


ILLUMINATION  OF  THE  SAMPLES 

Opaque  objects  such  as  metals  and  alloys  must  necessarily  be  examined  by  re- 
flected light,  i.e.  by  light  thrown  upon  them  from  above  the  stage,  their  treatment 
differing  in  this  respect  from  that  of  other  microscopic  preparations,  which  are  gen- 
erally examined  by  transmitted  light,  i.e.  by  light  sent  through  them  and  proceed- 
ing from  below  the  stage. 

With  the  low-power  objectives  there  are  two  possible  ways  of  illuminating  opaque 
specimens:  (1)  by  directing  the  light  obliquely  upon  the  object,  and  (2)  by  causing 
the  light  to  fall  perpendicularly  upon  it  by  means  of  so-called  "vertical  illuminators." 


Fig.  15.  —  Universal  mctalloseope :  mechanical  stage  on 
horseshoe  base,  magnetic  specimen  holder,  small  speci- 
men, and  base  plate. 

With  medium-high  and  high-power  objectives  the  second  method  only  is  possible, 
because  the  distance  between  the  specimen  and  the  front  lens  of  the  objective  is  now 
so  small  that  obliquely  reflected  light  cannot  reach  the  surface  under  examination. 
With  very  low-power  objectives  —  i.e.  having  a  focal  length  of  one  inch  or  more  — 
the  vertical  illuminator  may  be  placed  between  the  lens  and  the  object;  but  with 
higher  power  objectives  it  must  of  course  be  inserted  between  the  objective  and  the 
microscope  tube,  the  objective  then  acting  as  a  light  condenser  and  increasing  the 
intensity  of  the  illumination. 

Oblique  illumination  may  be  obtained  (a)  by  allowing  daylight  or  artificial  light 
to  fall  freely  upon  the  object;  (6)  by  directing  the  light  upon  the  object  by  means  of 
mirrors,  reflectors,  or  condensers;  (c)  by  the  use  of  a  " lieberkiihn " ;  and  (d)  by  the 
use  of  a  "parabolic  reflector." 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


15 


Vertical  illumination  may  be  produced  (a)  by  means  of  an  opaque  reflector  con- 
sisting of  a  totally  reflecting  prism  or  of  a  mirror  covering  only  a  portion  of  the  ob- 
jective, the  light  returned  by  the  object  reaching  the  eye  by  passing  through  the 
uncovered  portion;  and  (6)  by  means  of  a  transparent  reflector,  generally  a  plain 
glass  disk  or  glass  square,  reflecting  upon  the  object  a  portion  of  the  incident  light 
and  permitting  the  passage  of  a  portion  of  the  light  returned  by  the  object,  which 
thus  reaches  the  eye. 

When  a  highly  polished  surface  is  examined  by  obliquely  reflected  light,  since  the 
angle  of  reflection  is  equal  to  the  angle  of  incidence,  the  totality  of  the  light  is  reflected 
outside  the  objective  (Fig.  16)  and  the  object  appears  uniformly  dark.  In  case  the 
metallic  specimen  contains  some  portions  duller  in  appearance,~~these  will  scatter  a 
certain  amount  of  light  a  part  of  which  will  enter  the  objective  (Fig.  16),  and  those 
portions  will  therefore  appear  brighter.  A  similar  effect  is  produced  when  the  speci- 


(«)  (ft)  (c) 

Fig.  16.  —  (a)  Oblique  and  vertical  illuminations  of  bright  surface. 
(6)  Oblique  and  vertical  illuminations  of  dull  surface. 
(c)  Oblique  and  vertical  illuminations  of  hills  and  valleys. 

men,  instead  of  being  perfectly  flat,  contains  microscopic  hills  and  valleys,  the  sides 
of  which  may  be  so  inclined  as  to  reflect  some  light  into  the  microscope  (Fig.  16), 
consequently  appearing  bright.  Viewed  by  oblique  light,  therefore,  the  relative  dark- 
ness or  brightness  of  a  constituent  will  vary  inversely  with  its  true  appearance  and 
will  also  depend  upon  its  orientation,  since  this  will  affect  the  angle  of  incidence  of 
the  light  striking  it.  Generally  speaking,  the  darker  a  constituent  the  brighter  will 
it  seem  to  be  when  illuminated  by  oblique  light,  the  latter  yielding,  so  to  speak,  a 
negative  image.  Oblique  illumination,  moreover,  cannot  be  made  as  intense  as  ver- 
tical illumination  and,  as  already  explained,  is  possible  only  with  low-power  objectives. 
For  these  and  other  reasons,  while  it  is  not  without  value,  it  is  only  used  occasionally 
by  metallographists. 

To  increase  the  intensity  of  oblique  illumination  and  to  make  its  use  possible  with 
somewhat  higher  powers,  such  appliances  as  the  "lieberkiihn"  and  the  parabolic 
reflector  have  been  used.  The  "lieberkuhn,"  so  called  from  the  name  of  its  inventor, 
consists  of  a  small  concave  mirror  attached  to  the  objective  and  reflecting  upon  the 
object  some  light  proceeding  from  below  the  stage  and  passing  around  the  object. 
It  will  be  evident  that  only  small  size  objects  can  be  thus  illuminated. 


JO        APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 

The  parabolic  reflector  (Fig.  17),  first  constructed  by  Messrs.  Beck  of  London  for 
Dr.  Sorby,  consists  of  a  parabolic  mirror  placed  on  one  side  between  the  objective  and 
the  object  and  condensing  the  incident  light  upon  the  latter.  It  should  be  attached 
to  the  objective.  Dr.  Sorby  later  added  a  silver  mirror  in  the  shape  of  a  half  disk  to 
the  same  mount,  so  as  to  be  able  to  obtain  at  will  vertical  and  oblique  illumination 
when  using  low-power  objectives  (Fig.  17).  When  vertical  illumination  is  desired,  the 
small  mirror  is  swung  over  the  objective,  covering  only  a  portion  of  it,  and  directing 
vertical  rays  of  light  upon  the  object.  This  combination  is  known  as  the  Sorby-Beck 
reflector. 

The  effects  of  a  vertical  illumination  are  precisely  opposite  to  those  of  an  oblique 
illumination,  as  clearly  shown  in  Figure  16,  highly  polished  surfaces  reflecting  the  to- 
tality of  the  light  into  the  objective,  while  dull  ones  appear  dull  because  they  reflect 
most  of  the  light  outside. 

To  produce  a  vertical  illumination  we  have  the  choice  between  an  opaque  or  a 


(«)  (6) 

Fig.  17.  —  (a)  Parabolic  reflector. 

(6)  Sorby-Beck  parabolic  reflector. 

transparent  (glass)  reflector.  The  opaque  reflector  consists  of  a  totally  reflecting 
right-angled  prism,  or  of  a  mirror  placed  between  the  microscope  tube  and  the  objec- 
tive and  covering  only  a  portion  (generally  about  one  half)  of  its  aperture.  The  beam 
of  light  enters  the  illuminator  through  a  side  opening  provided  for  that  purpose  and 
is  reflected  downwards  by  the  reflector,  being  condensed  upon  the  object  by  the  lenses 
of  the  objective  itself.  The  light  sent  back  by  the  object  reaches  the  eye  through  the 
uncovered  part  of  the  objective. 

The  first  vertical  illuminator  was  designed  by  Professor  Hamilton  L.  Smith  of 
Hobart  College,  Geneva,  N.  Y.,  and  consisted  of  a  small  annular  silver  mirror 
(Fig.  18),  forming  an  angle  of  45°  with  the  axis  of  the  microscope,  the  light  reflected  by 
the  object  passing  through  the  central  opening  on  its  way  to  the  eye.  Semi-circular  mir- 
rors, similarly  mounted  and  partially  covering  the  objective  (Fig.  18),  have  been  used 
with  equal  satisfaction,  and  the  author  has  obtained  good  results  with  a  very  small 
central  mirror  (Fig.  18)  suitably  mounted,  reflecting  the  light  upon  the  central  por- 
tion of  the  objective  lenses,  and  permitting  the  returned  light  to  reach  the  eye 
through  the  free  space  surrounding  the  mirror. 

Instead  of  a  mirror,  a  totally  reflecting  right-angled  prism  may  be  used  as  shown 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


17 


in  Figures  18  and  19,  covering  half  of  the  aperture  of  the  objective.  The  prism  is 
so  mounted  that  it  can  he  rotated  around  its  horizontal  axis,  this  being  needed  in  order 
to  secure  the  best  illumination  of  the  sample.  Nachet,  of  Paris,  provides  his  prism 
with  an  additional  motion  permitting  it  to  cover  a  greater  or  smaller  portion  of  the 
objective.  These  reflecting  prisms  are  now  used  much  more  than  the  reflecting  mirrors. 


V 

(6)  (c)  (rf) 

Fig.  18.  —  (a)  Annular  mirror. 

(6)  Semi-circular  mirror. 

(c)  Central  mirror. 

(d)  Totally  reflecting  prism. 

(e)  Plain  glass  disk. 


In  1874  Nachet  constructed  for  the  International  Commission  of  the  Meter  some 
objectives  provided  with  totally  reflecting  prisms  as  permanent  parts  of  their  mount- 
ings. In  low-power  objectives  a  prism  was  placed  above  the  first  lens  (Fig.  20),  while 
with  higher  power  objectives  it  was  necessarily  inserted  above  the  double  or  triple 
lens  system.  These  objectives  are  called  illuminating  objectives.  This  arrangement, 


A  B 

Fig.  19.  —  Vertical  illuminator.    Totally  reflecting  prism.     Zeiss. 

however,  has  not  been  found  very  satisfactory  and  with  one  notable  exception  is  sel- 
dom used  by  metallographists. 

In  vertical  illuminators  having  a  transparent  reflector,  the  latter  consists  of  a  plain 
glass  disk  covering  the  whole  of  the  aperture  of  the  objective  (Figs.  18  and  21).  The 
incident  light  is  in  part  reflected  upon  the  object,  while  another  portion  passes  freely 
through  the  glass  reflector.  A  part  of  the  light  returned  by  the  object  is  again  re- 
flected by  the  glass  illuminator,  while  another  portion  passes  through  it  and  thus  reaches 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


the  eye.  The  glass  reflector  is  so  mounted  that  it  can  be  rotated  around  its  horizontal 
axis  (Fig.  21).  The  amount  of  light  permitted  to  enter  the  illuminator  may  be  regu- 
lated by  an  iris  diaphragm  attached  to  the  side  opening  or  independently  mounted 
and  placed  between  it  and  the  source  of  light,  or  by  a  revolving  sleeve  attached  to  the 
illuminator  and  provided  with  different  size  openings.  The  first  plain  glass  illumina- 
tor was  constructed  by  Mr.  Beck  of  London. 

With  very  low-power  objectives  it  is  preferable  to  place  the  vertical  illuminator 


Fig.  20.  —  Nachct  illuminating  objectives. 

between  the  objective  and  the  object,  attaching  it  to  the  former  in  some  suitable  way, 
as  for  instance  by  providing  the  lower  end  of  the  objective  with  a  society  screw  (see 
Fig.  6). 

While  the  author  is  well  aware  that  some  metallographists  of  note  prefer  the  prism 
to  the  plain  glass  type  of  vertical  illuminator,  in  his  opinion  the  plain  glass  reflector  is 
greatly  superior.  While  the  illumination  obtained  by  its  use  is  not  quite  as  intense, 
it  is  certainly  more  uniform  and  less  liable  to  produce  a  distortion  of  the  image. 

An  improved  construction  of  the  plain  glass  vertical  illuminator  is  illustrated  in 


Fig.  21.  —  Bausch  and  Lomb  plain  glass  disk  vertical  illuminator. 

Figure  21.  The  glass  reflector  is  inserted  into  a  brass  ring  which  on  the  side  opposite 
the  milled  head  is  screwed  into  the  wall  of  the  brass  mounting,  practically  doing  away 
with  the  frequent  breaking  of  the  glass  and  greatly  facilitating  its  cleaning.  The  milled 
head  is  large,  which  makes  it  possible  to  impart  a  more  delicate  motion  to  the  glass 
reflector. 

SOURCES  OF  LIGHT  AND  CONDENSERS 

The  illumination  of  opaque  objects  such  as  metals  and  alloys  requires  an  intense 
source  of  light,  especially  for  their  photography.  Daylight  and  ordinary  gas  or  oil 
flames  should  be  discarded  as  not  suitable  for  the  purpose,  the  sources  of  light  which 


APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY 


19 


have  been  found  most  satisfactory  being,  in  the  order  of  their  excellence,  intensity, 
and  decreasing  cost:  (1)  the  electric  arc  lamp,  (2)  the  Nernst  lamp,  and  (3)  the  Wels- 
bach  gas  lamp. 

The  Welsbach  lamp  outfits  (Figs.  22  and  23)  are  very  inexpensive  and  quite  satis- 


Fig.  22.  —  Welsbach  lamp  and  double-convex  condensing  lens. 

N 

factory  for  visual  examination  by  low-  and  medium  high-power  objectives.  In 
taking  photomicrographs,  however,  their  lack  of  intensity  necessitates  very  long 
exposures,  while  with  high-power  objectives  the  light  received  upon  the  camera  screen 


Fig.  23.  —  Welsbach  lamp  and  bull's  eye  condenser. 

is  so  faint  as  to  render  proper  focusing  of  the  object  a  very  difficult,  if  not  impossible, 
operation. 

Two  kinds  of  electric  arc  lamps  are  now  supplied,  one  with  large  carbons  (Fig.  24) 
and  a  smaller  one  with  carbons  measuring  only  J/£  inch  in  diameter  (Fig.  27).  The 
carbons  should  be  placed  at  right  angles,  as  this  arrangement  directs  the  maximum 
amount  of  light  into  the  condensers.  The  positive  or  horizontal  carbon  should  be 
cored  and  larger  than  the  vertical  or  negative  carbon.  While  automatic  feeding  of 


20 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


the  carbons  (Fig.  26)  is  a  valuable  feature,  it  is  not  by  any  means  essential,  as 
remarkably  effective  hand-feed  lamps  are  now  constructed  by  which  a  very  steady 
light  can  be  maintained  (Fig.  25).  Automatic  mechanisms,  moreover,  are  liable  to 
get  out  of  order  and  occasional  sudden  shiftings  of  the  light  are  difficult  to  entirely 
eliminate. 

The  large  carbon  lamp  yields,  of  course,  by  far  the  most  intense  illumination  and 


Fig.  24.  —  Large  arc  lamp  outfit. 

is  the  only  one  suitable  for  direct  projection  of  metallic  samples  upon  a  screen  for  public 
exhibition.  In  taking  photomicrographs  with  the  large  arc  lamp  the  needed  exposures 
are  often  instantaneous  and  seldom  exceed  five  or,  at  the  most,  ten  seconds.  The 
lamp  consumes  from  fifteen  to  twenty  amperes. 

The  small  arc  lamp  (Fig.  27)  is  very  satisfactory  for  visual  examination  and  is, 
of  course,  much  less  expensive.     It,  however,  requires  longer  exposures  when  photo- 


Fig.  25.  —  Hand-feed  arc  lamp. 


Fig.  26.  —  Automatic  feed  arc  lamp. 


graphing.  The  position  of  the  carbons  can  be  regulated  with  great  nicety  by 'inde- 
pendent adjustments,  thus  securing  a  very  uniform  light.  The  lamp  consumes  about 
five  amperes. 

The  Nernst  lamp  (Fig.  28)  is  used  successfully  by  many  microscopists  and  undoubt- 
edly affords  a  very  satisfactory  illumination  both  for  visual  examination  and  for  pho- 
tomicrography. In  taking  photographs,  exposures  of  ten  seconds  or  more  are  needed, 
according  to  the  magnification  and  the  character  of  the  specimen. 


APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY 


21 


Summing  up,  if  we  desire  a  cheap  and  convenient  form  of  illumination  for  visual 
examination  with  objectives  of  low  and  medium  high  power,  the  Welsbach  lamp  will 
be  found  in  every  way  satisfactory;  while  for  the  taking  of  photomicrographs  and  for 
examination  by  high-power  objectives  the  electric  arc  lamp  and  the  Nernst  lamp 
should  be  recommended,  bearing  in  mind  that  the  large  arc  lamp  will  yield  light  of 


Fig.  27.  —  Small  electric  arc  lamp,  bull's  eye  condenser,  and  rheostat. 

greatest  intensity  but  will,  on  the  other  hand,  be  much  more  costly.  When  neither 
gas  nor  suitable  electric  current  are  available,  an  acetylene  lamp  should  be  used, 
provided  tanks  of  acetylene  gas  can  readily  be  obtained. 

Condensers.  —  Some  kind  of  condensing  attachment  must  be  placed  between  the 
source  of  light  and  the  vertical  illuminator  so  that  a  large  portion  of  the  light  may  be 
utilized  and  a  beam  of  suitable  size  directed  into  the  illuminator.  In  the  case  of  light 


Fig.  28.  —  Nernst  lamp  and  special  hull's  eye  condenser  on  adjustable 

supports. 

proceeding  from  a  luminous  point  or  at  least  from  a  small  luminous  area,  as  for  instance 
with  the  electric  arc,  at  least  two  lenses  or  systems  of  lenses  are  needed,  one  system, 
PL  and  ML  (Fig.  29),  placed  near  the  so'urce  of  light,  to  collect  the  divergent  rays 
and  convert  them  into  a  parallel  beam,  and  a  second  lens  CL  placed  at  some  distance 
from  the  first,  to  convert  the  parallel  beam  into  a  converging  one.  The  vertical  illu- 
minator should  be  located  at  such  a  distance  from  the  condensing  lens  that  the  beam 


22 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


of  light  will  cover  a  little  more  than  the  opening  through  which  it  enters  the  illumi- 
nator. A  glass  cooling  cell  CC,  filled  with  distilled  water  or  some  other  suitable  liquid, 
should  be  placed  between  the  two  lenses  in  order  to  absorb  a  large  amount  of  heat  and 
thereby  prevent  injury  to  the  objective.  An  iris  diaphragm,  /,  should  also  be  used  to 
control  the  amount  of  light  entering  the  vertical  illuminator.  This  diaphragm  should 
be  placed  in  front  of  the  converging  lens  and  should  be  provided  with  clips  for  holding 
ground  and  colored  glasses.  These  various  parts  should  be  mounted  on  a  so-called 
"optical  bench"  B  upon  which  they  can  slide. 

With  a  large  luminous  body  such  as  the  Welsbach  mantle,  a  single  double-convex 
lens  (Fig.  22)  or  a  bull's  eye  condenser  (planoconvex)  (Fig.  23)  is  sufficient  to  collect 
and  condense  the  necessary  amount  of  light.  It  should,  of  course,  be  placed  at  the 
proper  distance  both  from  the  vertical  illuminator  and  from  the  source  of  light.  The 
use  of  an  iris  diaphragm  attached  to  the  lens  or  on  a  separate  mount  is  advisable, 
since  it  affords  a  ready  means  of  controlling  the  amount  of  light  admitted  into  the 
illuminator. 

Monochromatic  Light.  • —  The  different  lamps  described  above  all  yield,  of  course, 

CL  CC   PL  ML 


Fig.  29.  —  Condensing  lenses,  cooling  cell,  iris  diaphragm,  automatic  shutter, 

and  optical  bench. 

white  light,  and  since  the  correction  even  of  apochromatic  objectives  for  chromatic 
aberration  is  never  perfect,  it  is  evident  that  the  use  of  monochromatic  light  —  i.e. 
light  of  one  wave  length  —  is  preferable,  especially  for  photographing.  Monochromatic 
light  may  be  obtained  in  two  ways:  (a)  by  using  a  source  of  light  actually  monochro- 
matic, and  (b)  by  causing  white  light  to  pass  through  colored  glass  screens  or  colored 
solutions  (light  filters),  preventing  the  passage  of  some  undesirable  rays.  The  mercury 
arc  lamp  yields  a  nearly  monochromatic  light  and  has  been  tried  by  Le  Chatelier  with 
satisfactory  results.  It  seems  more  convenient,  however,  when  monochromatic  light 
is  wanted,  to  use  light  filters  of  suitable  colors,  in  which  cat,e  colored  glass  screens  will 
be  found  easier  to  manipulate  than  glass  cells  containing  colored  solutions. 


PHOTOMICROGRAPHIC   CAMERAS 

For  taking  photomicrographs,  a  light-tight  connection  should  be  established  be- 
tween the  microscope  and  camera,  both  instruments  being  placed  vertically  or  hori- 
zontally (Figs.  30  and  31).  Whether  to  use  the  camera  in  a  vertical  or  horizontal 
position  is  a  debatable  and  debated  question.  Among  well-known  metallographists, 
Le  Chatelier  and  Martens  prefer  the  horizontal  while  Osmond  and  Stead  have  a  strong 


APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY  23 


Fig.  30.  —  Photomicrographic   camera  (vertical  position),  showing  metal- 
lurgical microscope,  mechanical  stage,  automatic  shutter,  etc. 


24 


APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY 


5 


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APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY 


25 


26 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


preference  for  the  vertical  position.  The  latter  certainly  affords  greater  stability  and 
eliminates  all  danger  of  heavy  specimens  slipping  while  being  photographed.  It  is 
often  said  that  the  vertical  position  is  inconvenient  because  of  the  necessity  of  mount- 
ing on  a  stool  for  focusing  the  image  on  the  screen,  but  the  objection  appears  trifling. 
It  is  also  argued  by  some  that  the  microscope  should  be  used  in  a  horizontal  position 
because  this  makes  it  possible  for  the  operator  to  sit  while  performing  his  work.  This 
objection  also  appears  to  have  little  weight. 

By  the  use  of  a  large,  totally  reflecting  prism,  however,  connecting  the  microscope 


Fig.  33.  —  Plan  view  of  metallurgical  microscope  and  photomicrographic  camera  joined  by 

totally  reflecting  prism;  large  arc  lamp  outfit  and  Welsbach  outfit  No.  5. 
Legend:    C  =  camera. 

B  =  camera  base  in  reversed  position. 

F  =  Fine  adjustment  of  microscope. 

F'=  Pulley  for  controlling  F  from  the  position,  1,  of  the  observer  at  the  camera. 

L  =  Arc  lamp  outfit. 

1  =  Iris  diaphragm. 

P  =  Horizontal  vertical  attachment. 
S  =  Automatic  shutter  with  iris  diaphragm. 

W  =  Welsbach  lamp  arranged  for  visual  examination  at  same  microscope. 
D  =  Condenser  for  Welsbach  lamp. 

2  =  Position  of  observer  at  microscope. 
A.  =  Adjustment  for  arc  lamp  carbons. 

and  camera  (Figs.  32  and  33),  it  is  possible  to  maintain  the  microscope  vertical,  un- 
doubtedly the  best  position,  while  using  the  camera  horizontally.  A  thread  belt  con- 
nects the  fine  adjustment  of  the  microscope  with  a  pulley  mounted  on  a  standard 
at  the  end  of  the  camera  bed  bar.  This  enables  the  operator  to  control  the  fine  ad- 
justment easily  from  his  position  at  the  camera  screen,  while  the  placing  of  the  arc 
lamp  on  the  same  side  of  the  microscope  as  the  camera  makes  it  possible  for  him  to 
reach  with  his  right  hand  the  various  adjustments  of  the  lamp,  thus  securing  max- 
imum intensity  and  uniformity  of  illumination,  two  points  so  essential  in  taking 
photomicrographs. 

For  visual  examination,  the  front  standard  of  the  camera  to  which  the  special  prism 


APPAKATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


27 


Fig.  34.  —  Universal  metalloscope,  Nernst  lamp  outfit,  and  vertical  camera. 


Fig.  35.  —  Universal  metalloscope,  arc   lamp   outfit,    large  totally   reflecting 
prism,  and  horizontal  camera. 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 

is  attached  is  moved  along  the  bed  bar  to  a  sufficient  distance.  To  bring  the  camera 
into  position  for  photography  the  front  standard  is  brought  towards  the  microscope 
until  the  collar  beneath  the  prism  rests  upon  the  eye-piece  of  the  microscope,  the  focus 
of  the  microscope  not  being  changed  in  the  least.  The  bottom  of  this  collar  which  is 
fitted  with  a  felt  rim  rests  directly  upon  the  eye-piece  without  exerting  any  pressure, 
but  tightly  enough  to  make  a  light-tight  connection.  The  image  is  then  focused  upon 
the  camera  screen  by  the  fine  adjustment  alone  and  the  exposure  quickly  made. 

The  universal  metalloscope  already  described  is  seen  in  Figure  34  with  vertical 
camera  and  Nernst  lamp  and  in  Figure  35  with  horizontal  camera,  totally  reflecting 
prism  and  large  arc  lamp. 

INVERTED  MICROSCOPES 

Le  Chatelier  was  the  first  to  suggest  the  use  of  an  inverted  microscope  for  the  ex- 
amination of  metallic  surfaces  (see  Appendix).  In  this  style  of  microsope  the  stage 
is  placed  horizontally  above  the  objective,  the  latter  being  necessarily  pointed  up- 
wards (Figs.  36  to  38). 

In  the  inverted  type  of  microscope  and  photographic  attachment  illustrated  in 
these  pages,  it  has  been  attempted  to  simplify  the  construction  with  corresponding 
material  decrease  in  price. 

The  microscope  is  permanently  connected  with  the  camera  by  a  totally  reflecting 
prism  P  (Fig.  37)  set  rigidly  below  the  vertical  illuminator.  A  separate  tube  set  at 
right  angles  to  the  first  is  provided  for  visual  examination,  another  totally  reflecting 
prism  P'  (Figs.  37  and  38)  being  fastened  to  the  inner  end  and  serving  to  reflect  the 
image  from  the  body  tube  through  the  eye-tube  to  the  eye.  When  a  photograph  is 
to  be  taken  this  prism  P'  is  simply  withdrawn  from  the  field  by  means  of  the  draw 
tube.  The  eye-tube  is  fitted  with  pin  and  slot  which  mark  the  limits  to  which  the 
small  prism  P'  may  be  pushed  in  and  withdrawn,  so  that  the  vertical  illuminator  being 
once  set,  the  only  adjustment  necessary  is  at  the  arc  lamp.  With  the  Nernst  and  Wels- 
bach  lamps,  after  the  light  and  the  vertical  illuminator  are  once  set,  no  more  adjust- 
ments are  necessary.  The  two  totally  reflecting  prisms  need  never  be  rotated  and  in 
fact  cannot  be  moved,  except  for  the  sliding  motion  of  the  prism  P'  as  already  described. 

The  stage,  which  is  revolving  and  provided  with  centering  screws,  is  of  course 
equipped  with  both  coarse  and  fine  adjustment,  and  a  mechanical  stage  may  readily 
be  substituted  for  the  plain  stage. 

With  this  inverted  microscope  the  use  of  a  magnetic  holder  will  also  be  found  very 
convenient,  for  the  sample  is  then  held  firmly  in  place  instead  of  resting  loosely  on 
the  stage,  thereby  increasing  the  usefulness  of  the  mechanical  stage. 

The  placing  of  the  light  on  the  same  side  of  the  microscope  as  the  camera  makes 
it  possible  for  the  operator  to  regulate  his  illumination  while  focusing  the  object  on 
the  camera's  screen. 

POLISHING  APPARATUS 

Hand  Polishing.  —  When,  in  spite  of  the  length  and  laboriousness  of  the  operation, 
iron  and  steel  samples  are  to  be  polished  by  hand,  four  smooth  blocks  of  wood  should 
be  prepared,  some  6  by  10  inches  and  1  inch  thick.  One  of  these  should  lie  covered 
with  canvas  or  linen  duck  and  the  others  with  fine  broadcloth.  The  blocks  are  to  be 
used  as  described  in  Lesson  III,  page  5. 


APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY 


2!) 


3 
O 

&. 

a 


be 

jg 

•a 


| 


K 


30 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


Polishing  by  Power.  —  The  power  polishing  machine  shown  in  Figure  39  has  been 
found  very  satisfactory.  It  consists  of  a  heavy  iron  pedestal  upon  which  is  mounted 
a  grinder  having  emery-wheel  and  cast-iron  disks  revolving  in  a  vertical  plane,  thus 
giving  four  polishing  surfaces  of  graduated  fineness.  The  polishing  powders  mixed 
with  water  are  applied  to  the  various  disks  by  means  of  brushes,  and  shields  are  pro- 
vided to  catch  any  surplus  water  that  may  be  thrown  off  during  the  operation.  Should 
a  cloth  become  worn  or  torn  it  is  readily  and  quickly  replaced.  This  machine  very 
much  shortens  the  time  necessary  for  the  preparation  of  samples  and  is  far  superior 
to  those  where  only  one  block  is  made  to  rotate  at  a  time.  A  speed  of  1200  revolutions 
per  minute  has  been  found  most  satisfactory  for  polishing  iron  and  steel  samples,  but 


Fig.  37.  Fig.  38. 

Fig.  37.  —  Inverted  metalloscope.    Vertical  section,  front  view. 
Fig.  38.  —  Inverted  metalloscope.    Vertical  section,  end  view  on  AB  (Fig.  37). 

L  =  Source  of  Light. 

R  =  Vertical  illuminator  reflector.1 

P'  =  Totally  reflecting  prism  which  reflects  image  into  the  eye-tube  when  latter  is  pushed 
in. 

P  =  Totally  reflecting  prism  which  reflects  image  into  camera  when  eye-tube  is  pulled  out. 

S  =  Metalloscope  stage. 

O  =  Specimen. 

by  the  use  of  a  variable  speed  electric  motor  to  run  the  polishing  machine  various 
speeds  may  be  readily  obtained. 

The  polishing  motor  shown  in  Figure  40  possesses  the  advantages  of  directly  driven 
over  belt  driven  machinery.  It  is  provided  with  the  same  polishing  disks  as  the  pol- 
ishing machine  and  can  be  built  both  for  constant  and  for  variable  speed. 

The  operation  of  polishing  with  these  machines  is  described  in  Lesson  III,  page  6. 


PYROMETERS  AND   ELECTRIC   FURNACES 

Pyrometers.  —  The  Le  Chatelier  thermo-electric  pyrometer  is  undoubtedly  the 
instrument  best  adapted  to  the  measurement  of  temperatures  needed  to  control  such 
heat  treatments  as  are  likely  to  be  performed  in  a  metallographical  laboratory.  The 
thermo-couple  consists  of  a  wire  of  pure  platinum  and  of  a  wire  of  platinum  alloyed 
with  10  per  cent  of  rhodium  or  iridium.  To  measure  the  electromotive  force  created 

1  A  reflecting  prism  may  of  course  be  used  if  preferred. 


APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY  31 


Fig.  39.  —  Power  polishing  machine. 


Fig.  40.  —  Polishing  motor. 


32 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


by  heating  the  couple  the  author  does  not  know  of  any  more  accurate  and  reliable 
instrument  than  the  galvanometer  constructed  by  Siemens  and  Halske   (Fig.  41). 


Fig.  41.  —  Siemens  and  Halske  galvanometer. 


Pt.— Rh. 
Fig.  42.  —  Saladin  self-recording  thermo-electric  pyrometer. 

In  its  latest  form  it  has  a  resistence  of  400  ohms,  has  a  scale  of  180  divisions,  each 
one  corresponding  to  10  microvolts  and  a  second  graduation  giving  the  temperature 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


33 


din>ctly  for  the  couple  sold  with  the  apparatus.  The  use  of  cheaper  couples  and 
cheaper  galvanometers  is  not  to  be  commended  for  they  are  unsuitable  for  accu- 
rate scientific  work  and  uncertain  even  for  controlling  industrial  operations. 

An  autographic  recording  pyrometer  is  very  useful  and  quite  indispensable  for 
the  detection  of  faint  evolutions  or  absorptions  of  heat.     Indeed  without  its  use  there 


Fig.   43.  —  Le   Chatelier-Saladin    self-re- 
cording thermo-electric  pyrometer. 


Fig.  44.  —  Le  Chatelier-Saladin  self-recording  thermo-electric  pyrometer. 

are  many  delicate  thermal  treatments  that  could  not  be  performed.  Several  auto- 
graphic instruments  are  now  constructed.  To  meet  the  needs  of  the  metallographist 
the  author  believes  that  the  pyrometric  recorder  designed  by  Le  Chatelier  and  Sala- 
din  and  constructed  by  Pellin  of  Paris  (Figs  42  to  45)  will  be  found  most  satisfactory. 
In  an  early  form  the  different  parts  were  arranged  as  shown  in  Figure  42.  The  light 
proceeding  from  the  source  S  after  passing  through  a  lens  is  received  by  the  mirror 
of  a  sensitive  galvanometer  Gi  the  deflections  of  which  measure  the  difference  in 


34 


APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 


temperature  between  the  sample  under  examination  and  the  neutral  body.  This 
horizontal  deflection  of  the  beam  of  light  is  converted  into  a  vertical  deflection  by 
passing  through  a  totally  reflecting  prism  M  placed  at  an  angle  of  45  deg. 

This  vertically  moving  beam  of  light  is  received  by  the  mirror  of  a  second  gal- 
vanometer Gi  whose  deflections  are  a  measure  of  the  temperature  of  the  sample.   The 

Ft 


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1 

1 

2~     --   .     .  .'.   .  ''//•//'      /,                 '    '                  '   '/    '/                 -'"    '      ''    ' 

^J  Laourup  Curves 

7  .^»w^w.ir,.i0%,X  M^SM^I 

IV10%lr.    <  — 

*  

7  :  ;  .  -,  '  •  -\  N  1   "  —  ^SS  *•'  '  x^l% 

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C                                         H 

•''-•••                                      ,   •  /  ',;/  -/,    ,  '  >,  ;•/• 

Sensitive 
Galvanometer 


Fig.  45.  —  Roberts-Austen  method  of  connecting  the  sample,  neutral  body, 
and  galvanometers. 


Fig.  46.  —  Herseus  electric  muffle  furnace. 

beam  then  passes  through  a  lens  and  reaches  the  screen  or  plate  P.  L  is  a  lens  at  t he- 
conjugate  foci  of  which  are  placed  the  mirrors  of  the  two  galvanometers.  Two 
motions  are  in  this  way  imparted  to  the  spot  of  light,  (1)  a  horizontal  motion  pro- 
portional to  the  temperature  of  the  sample  and  (2)  a  vertical  motion  proportional  to 
the  difference  in  temperature  between  the  sample  and  the  neutral  body.  The  result- 
ing curves  are  known  as  differential  curves  (see  Lesson  VII,  pages  10  and  seg.) 

In  recent  models  the  apparatus  has  been  simplified  and  made  more  compact 


APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY  35 

(Figs.  43  and  44)  by  placing  the  galvanometers  so  near  each  other  that  the  lens  L, 
Figure  42,  could  be  omitted  and  the  entire  instrument  placed  in  a  metallic  or  wooden 
case  (Fig.  44.) 

The  connections  between  the  sample  and  the  neutral  body  and  between  these 
and  ftie  galvanometers  are  made,  as  first  suggested  by  Roberts-Austen,  and  as  clearly 
shown  in  Figure  45.  In  this  illustration  galvanometer  GI  corresponds  to  galvanom- 
eter G-2  of  Figure  42  and  galvanometer  Gz  to  galvanometer  GI. 


ELECTRIC    FURNACES 

For  the  experimental  heat  treatment  of  small  iron  and  steel  samples  an  electric 
resistance  furnace  is  extremely  useful.  For  such  purposes  the  Heraus  platinum 
wound  muffle  furnaces  (Fig.  46)  are  very  satisfactory.  The  furnaces  are  provided 
with  a  safety  device  to  take  care  of  any  overload  and  with  an  internal  adjustable 
rheostat.  They  are  made  in  different  sizes,  the  inside  measurements  of  the  largest 
type  being  nine  inches  in  length,  six  inches  in  width,  and  three  and  one  half  inches  in 
height.  Using  110  volt  current  they  consume  from  five  to  fifteen  amperes  according 
to  size  and  temperature.  If  care  be  taken  never  to  use  temperatures  exceeding  1000 
deg.  C.  for  any  great  length  of  time,  and,  preferably,  never  to  exceed  1000  deg.,  the 
furnace  will  be  found  very  durable.  The  use  of  cheaper  furnaces  wound  with  base 
metals  is  a  doubtful  economy  as  frequent  rewindings  are  required  while  the  maxi- 
mum temperature  that  can  be  safely  used  with  such  furnaces  is  fully  200  deg.  C. 
lower  than  with  platinum  resistance  furnaces. 


-- 


LESSON   I 

PURE  METALS 

Microstructure.  —  When  a  properly  prepared  sample  of  a  pure  metal  is  examined 
under  the  microscope,  the  revealed  structure  generally  presents  the  appearance  of  a 
polygonal1  network  (Figs.  1  and  2),  an  indication  that  the  metal  itself  is  composed  of 


Fig.  1.  —  Pure  gold.     Cast. 
Magnified  50  diameters.     (Andrews.) 


Fig.  2.  —  Pure  copper. 

Magnified  8  diameters. 

(Houghton.) 


irregular  polyhedral2  grains,  each  mesh  or  polygon  of  the  network  representing  a 
section  through  a  polyhedron.2 

Crystallization.  —  When  a  substance  passes  from  the  liquid  to  the  solid  state,  the 
process  of  solidification  is  generally  accompanied  by  crystallization,  i.e.  the  molecules 
of  the  substance  so  arrange  themselves  as  to  form  small  solids  of  regular  geometrical 
outlines,  such  as  cubes,  octahedra,3  etc.  Each  of  these  spontaneously  formed  sym- 
metrical solids  is  called  a  crystal  and  any  substance  made  up  of  crystals  is  said  to  be 
crystalline. 

Crystals  possess  the  property  of  breaking  more  readily  in  one  or  more  directions. 
This  property  is  called  "cleavage."  The  planes  of  cleavage  or  direction  of  easy  rup- 
ture are  generally  parallel  to  the  faces  of  the  crystals.  A  cubic  crystal,  for  instance, 

1  A  polygon  is  a  closed  geometrical  figure  with  straight  sides  (necessarily  three  or  more). 

2  A  polyhedron  (plural,  polyhedra  or  polyhedrons)  is  a  closed  geometrical  solid  bounded  by 
plane  (smooth)  faces  (necessarily  four  or  more). 

3  An  octahedron  (plural,  octahedra  or  octahedrons)  is  a  geometrical  solid  (a  polyhedron)  bounded 
by  eight  plane  faces. 


2  LESSON  I  — PURE  METALS 

splits  readily  in  three  planes  parallel  to  the  three  sets  of  faces  of  the  solid.  In  Figure  3, 
ABC,  DEF,  and  GHI  indicate  the  cleavage  planes  of  a  cubic  crystal.  The  direction  of 
its  cleavage  planes  constitutes  the  orientation  of  the  crystal. 

Solid  substances  which  are  not  crystalline  are  said  to  be  "amorphous."  They  are 
characterized  by  the  absence  of  any  symmetrical  grouping  of  their  molecules.  Glass 
is  a  good  example  of  an  amorphous  substance. 

Idiomorphic  Crystals.  —  When  the  fluidity  of  a  substance  and  other  conditions  are 
such  that  the  formation  and  growth  of  the  crystals  are  given  free  play,  perfect  (and 


Fig.  3.  —  Cleavage  planes. 
(Mellor.) 


Fig.  4.  —  Crystallization  from 
centers.     (Desch.) 


sometimes  very  large)  crystals  are  produced.  These  perfect  crystals,  with  faultless 
geometrical  outlines,  perfect  cubes  for  instance,  are  called  "idiomorphic"  crystals. 

Allotrimorphic  Crystals.  —  When  the  free  development  of  crystals  is  hindered  by 
less  favorable  crystallizing  conditions,  such  for  instance  as  collision  or  contact  with 
other  crystals,  likewise  in  process  of  formation,  the  regular  external  form  is  not  pre- 
served and  the  resulting  imperfect  crystals  are  called  "  allotrimorphic  "  crystals,  also, 
but  more  seldom,  "anhedrons"  or  faceless  crystals.  Such  crystals  are  said  to  have 
taken  their  shape  from  their  surroundings.  It  should  be  noted,  however,  that  allotri- 
morphic crystals,  like  idiomorphic  crystals,  are  composed  of  crystalline  matter.  An 
allotrimorphic  crystal  may  be  regarded  as  resulting  from  the  mutilation  of  an  idio- 
morphic crystal,  the  mutilation  affecting  the  external  shape  only,  and  not  the  crystal- 
line character  of  the  substance.1 

Crystallization  of  Metals.  —  Metals  when  they  solidify  generally  give  rise  to  the 
formation  of  allotrimorphic  crystals.  The  explanation  for  this  is  to  be  found  in  the 
fact  that  crystallization  sets  in  simultaneously  at  many  different  centers  (see  Fig.  4). 
From  each  center  a  crystal  grows  through  successive  addition  of  crystalline  matter 
similarly  oriented,  until  meeting  with  other  surrounding  crystalline  growths,  radiating 
from  other  centers,  the  free  development  of  its  external  form  is  arrested.  The  polyg- 
onal networks  shown  in  Figures  1  and  2,  representing  the  structure  respectively  of 
pure  gold  and  pure  copper,  do  not  indicate,  therefore,  cleavage  planes,  i.e.  outlines  of 
true  crystals,  but  merely  boundaries  or  junction  lines  between  adjacent  crystalline 
growths  or  "grains."  They  mark  the  regions  where  neighboring  crystalline  growths 
collided  with  resulting  distortion  of  their  external  forms. 

1  Crystalline  groups  or  aggregates  of  allotrimorphic  crystals  are  sometimes  called  "crystallites" 
while  if  they  assume  some  distinct  form  they  may  be  further  described  as  "dendrites"  or  "tree- 
like," "fern  leaves,"  "star-like"  crystallites,  "crystalline  grains,"  etc. 


LESSON   I  — PURE   METALS  3 

Grains  of  Metals.  —  If  crystallographers  were  interested  in  the  constitution  and 
structure  of  metals  they  would  undoubtedly  refer  to  the  small  polyhedral  grains  of 
which  they  are  composed  as  allotrimorphic  crystals.  This  expression,  however, 
although  the  only  scientifically  correct  one,  does  not  appeal  to  metallurgists,  who 
generally  call  these  imperfect  crystals  "crystal  grains,"  "crystalline  grains,"  or  even 
simply  "grains."  The  expression  crystalline  grains  which  is  quite  common  appears 
satisfactory  since  it  suggests  the  two  main  facts  to  be  remembered  as  to  the  nature 
of  the  grains,  (1)  that  they  are  not  perfect  crystals  and  (2)  that  they  are  nevertheless 
crystalline.  Dana  himself  writes  that  cast  iron  is  made  up  of  crystalline  grains. 
He  says  further:  "Crystallization  produces  masses  made  of  crystalline  grains  when 
it  cannot  make  distinct  crystals."  Nor  is  there  very  great  objection  if,  for  the  sake 
of  brevity,  the  word  grain  alone  be  used,  bearing  in  mind  once  for  all  the  crystalline 
character  of  metals. 


Fig.  5.  —  Pure  platinum. 

Cast.     Magnified  120  diameters. 

(Andrews.) 

To  render  the  polygonal  boundaries  of  crystalline  grains  visible  under  the  micro- 
scope the  highly  polished  metallic  surface  must  generally  be  treated  by  an  acid  or 
some  other  chemical  reagent  capable  of  dissolving  or  corroding  the  metal,  with  or 
without  deposition  of  a  film  of  some  precipitated  matter.  According  to  Ewing  these 
boundaries  are  made  evident  by  the  differential  action  of  the  acid  which  produces 
differences  of  level  by  attacking  one  grain  more  energetically  than  its  neighbors. 
Each  of  the  short  sloping  surfaces  which  connect  one  grain  with  another  appears 
black  under  vertical  illumination  because  it  does  not  reflect  the  light  back  into  the 
tube  of  the  microscope. 

Crystalline  Orientation  of  the  Grains.  —  A  slight  chemical  attack  or  etching  of 
the  polished  samples  brings  out  merely  the  polygonal  structure  described  and  illus- 
trated in  the  preceding  pages.  If  the  etching  be  somewhat  deeper,  however  (through 
the  use  of  a  stronger  reagent  or  because  of  a  longer  attack),  it  is  observed  that  the 
polygons  or  meshes  of  the  network  are  differently  colored  (Fig.  5),  some  appearing 
very  dark,  others  less  dark,  others,  still,  brilliant.  This  heterogeneousness  in  the 
appearance  of  sections  through  adjacent  grains  is  due  to  the  fact  elucidated  above 
that  each  grain,  i.e.  each  allotrimorphic  crystal,  is  made  up  of  crystalline  matter  similarly 


4  LESSON   I  — PURE  METALS 

oriented  in  the  same  grain  but  differently  in  different  grains.  Bearing  this  in  mind 
the  dissimilarity  of  coloration  between  contiguous  grains  is  readily  explained,  for  if 
the  crystalline  matter  of  any  individual  grain  is  so  oriented  that  it  reflects  the  in- 
cident light  into  the  microscope  tube,  that  grain  will  appear  bright,  while,  on  the 
contrary,  if  its  orientation  is  such  that  the  light  is  reflected  outside  the  microscope, 
the  corresponding  grain  will  appear  dark.  By  slightly  inclining  the  sample  in  various 
directions,  or  by  rotating  it,  some  of  the  grains  that  were  bright  become  dark,  being, 
so  to  speak,  extinguished,  while  some  of  the  dark  grains  become  brightly  illuminated, 
because  in  so  doing  we  change  the  direction  of  the  light  reflected  by  each  individual 
grain  section.  Similar  results  are  obtained  by  changing  the  direction  of  the  incident 
light.  The  kaleidoscopic  effect  just  described  affords  a  conclusive  proof  of  the  crys- 
talline nature  of  metals  and  of  the  correctness  of  the  explanation  offered  to  account 
for  the  dissimilar  appearance,  as  to  color,  of  contiguous  grains. 

Cubic  Crystallization  of  Metals  —  Etching  Pits.  —  By  still  deeper  etching  of  the 
polished  surfaces  of  pure  metals,  it  is  sometimes  possible  to  bring  out  clearly  the 
crystalline  character  of  the  individual  grains  (see  Fig.  6).  The  figures  thus  outlined, 


Fig.  6.  —  Typical  etching 

figures  of  pure  metals. 

(Gulliver.) 


in  reality  small  cavities,  are  often  called  "etching  pits"  or  "etching  figures."  These 
figures  frequently  correspond  to  sections  of  cubes  or  of  geometrical  solids  derived 
from  the  cube,  indicating  that  most  metals  crystallize  in  the  cubic  system  (also  called 
regular,  or  isometric,  or  monometric  system).1 

Summary.  —  Summing  up  the  indications  obtained  by  the  microscopical  exam- 
ination of  polished  and  etched  surfaces  of  pure  metals,  it  has  been  shown  (1)  that  a 
slight  etching  outlines  the  polygonal  boundaries  of  adjacent  crystalline  grains,  (2) 
that  a  deeper  etching  imparts  different  colorations  to  the  various  polygons  or  grain 
sections,  a  phenomenon  which  is  due  to  the  constancy  of  crystalline  orientation  in 
any  individual  grain  and  to  the  change  of  orientation  as  we  pass  from  one  grain  to 
the  next,  and  (3)  that  a  still  deeper  attack  often  brings  out  clearly  pits  of  distinct 
geometrical  forms,  often  cubic,  indicating  that  the  majority  of  metals  crystallize  in 
the  cubic  system.2 

1  The  other  crystallographic  systems  are  the  hexagonal,  the  tetragonal,  the  orthorhombic,  the 
monoclinic,  and  the  triclinic. 

2  We  have  other  indications  of  the  cubic  crystallization  of  metals  such  for  instance  as  the  hex- 
agonal character  of  many  of  the  polygons  which  crystallographers  consider  to  be  due  to  interfering 
cubes  and  octahedra  (the  octahedron  is  a  form  belonging  to  the  cubic  system).    Again,  under  favor- 
able crystallizing  conditions  nearly  perfect  cubes  have  been  obtained  in  the  case  of  several  metals. 


LESSON   I  — PURE   METALS  5 

Impurities.  —  It  is  well  known  that  the  addition  of  surprisingly  small  amounts  of 
impurities  or  foreign  substances  often  affect  very  greatly  some  of  the  most  important 
properties  of  metals,  such  as  their  strength,  ductility,  fusibility,  electrical  conduc- 
tivity, etc.,  and  we  naturally  look  for  correspondingly  marked  changes  of  structure. 
In  order  to  understand  this  important  influence  of  impurities  upon  the  properties  of 
metals  it  will  be  necessary  to  consider  at  some  length  the  nature  of  the  union  which 
exists  between  the  metal  and  the  impurity.  Let  us  first  note  that  by  impurity  we 
mean  a  very  small  proportion  of  some  foreign  substance  which  may  be  any  other 
metal,  a  metalloid,  or  a  definite  compound. 

The  metal  or  metalloid  contaminating^  the  metal  may  (1)  remain  uncombined  or 
(2)  it  may  combine  with  some  (generally  a  small  amount)  of  UTe^metal  to  form  a 
definite  chemical  compound.  The  uncombined  contaminating  metal  or  metalloid 
or  resulting  chemical  compound  may  then  (a)  be  soluble  in  the  solid  metal  forming 
with  it  a  "solid  solution"  or  (6)  be  insoluble  in  the  solid  metal  in  which  case  it  is 
rejected  by  the  crystalline  grains,  in  the  form  of  an  eutectic  alloy. 

The  meaning  of  the  expressions  "solid  solutions"  and  "eutectic  alloys"  should 
now  be  explained.  As  Professor  Howe  has  well  expressed  it  the  essential  features  of 
an  ordinary  liquid  solution  are  (1)  a  complete  merging  of  the  constituents  and  (2)  in 
indefinite  proportions.  By  complete  merging  is  meant  a  union  so  intimate  that  the 
separate  existence  of  the  constituents  cannot  be  detected  by  any  physical  means, 
such  for  instance  as  microscopic  examination  under  the  highest  magnification.  The 
homogeneity  of  the  substance  is  such  that  it  resists  any  physical  attempt  at  breaking 
it.  The  merging  moreover  must  remain  complete  and  absolute  for  varying  propor- 
tions of  the  constituents,  for  it  is  evident  that  if  it  existed  only  for  certain  well-defined 
proportions  of  the  component  parts,  the  resulting  substance  would  be  of  the  nature 
of  a  definite  chemical  compound  and  not  of  a  solution. 

Bearing  in  mind  these  characteristics  of  ordinary  solutions,  we  find  that  in  some 
substances,  while  passing  from  the  liquid  to  the  solid  state,  the  constituents  remain 
completely  merged  and  in  indefinite  proportions.  The  essential  characteristics  of 
liquid  solutions  are  retained  in  the  solid  state.  Hence  the  name  of  solid  solutions  given 
to  such  substances.  A  common  and  excellent  example  of  solid  solutions  is  found  in 
the  case  of  glass  in  which  the  three  usual  constituents,  silica,  lime,  and  alkali,  are  so 
completely  merged  that  their  existence  cannot  be  detected  by  physical  means;  the 
microscopical  examination  of  glass  under  the  highest  magnification  fails  to  reveal  the 
presence  of  its  component  parts.  Glass  on  solidifying  passes  from  the  condition  of  a 
liquid  solution  to  that  of  a  solid  solution.  Many  metals  likewise  form  on  solidifying 
solid  solutions;  i.e.  they  solidify  into  a  mass  so  absolutely  homogeneous  that  the 
identity  of  the  component  metals  is  entirely  lost.  The  union  between  some  metals 
and  metalloids  also  frequently  forms  solid  solutions. 

It  is  held  by  some  crystallographers  that  in  order  to  form  solid  solutions  the  uni- 
ting substances  must  be  "isomorphous,"  that  is,  must  be  capable  of  yielding  crystals 
of  the  same  form,  hence  the  name  of  "isomorphous  mixtures"  frequently  given  to 
solid  solutions.1  The  homogeneous  crystals  formed  by  solid  solutions  are  often  called 

1  If  isomorphism  favors  the  formation  of  solid  solutions,  as  it  undoubtedly  does,  seeing  that 
most  metals  are  isomorphous,  we  naturally  infer  that  they  will  readily  form  solid  solutions.  We 
now  know  that  such  is  the  case,  for  if  metals  are  not  generally  soluble  in  each  other  (when  solid)  in 
all  proportions  there  are  few  instances  of  metals  entirely  insoluble  in  each  other  in  the  solid  state. 
The  formation  of  solid  solutions  between  metals  is  therefore  very  frequent. 


6  LESSON   I  — PURE   METALS 

"mixed  crystals"  —  and  that  expression  frequently  used  as  an  equivalent  for  solid 
solution.  There  are  some  crystallographers,  however,  who  believe  that  isomorphism 
of  the  constituents  is  not  essential  to  the  formation  between  them  of  solid  solutions, 
and  that  the  use,  therefore,  of  isomorphous  mixtures  as  synonymous  of  solid  solu- 
tion is  not  warranted.  The  use  of  the  expression  mixed  crystals  is  likewise  to  be 
discouraged  because  it  suggests  a  mixture,  and,  therefore,  heterogeneity,  which  is 
precisely  contrary  to  the  nature  of  solid  solutions. 

Considering  now  those  impurities,  whether  metals,  metalloids,  or  definite  com- 
pounds, which  form  solid  solutions  with  the  metal  they  contaminate,  it  is  found 
as  might  have  been  expected  that  their  presence  has  no  great  influence  upon  the 
character  of  the  structure.  Suitably  prepared  surfaces  of  such  impure  metals  still 
exhibit  the  polygonal  network  structure  characteristic  of  pure  metals.  The  small 
polyhedra  of  which  the  impure  metal  is  composed,  however,  are  now  allotrimorphic 


Fig.  7.  —  Gold  containing  0.20  per  cent 
lead.     Magnified  100  diameters. 

(Andrews.) 

crystals  of  a  solid  solution  instead  of  a  pure  metal.  While  the  character  of  the  struc- 
ture remains  the  same,  the  dimension  of  the  grains  may  be  markedly  affected,  being 
often  increased  by  the  presence  of  a  small  amount  of  impurity  forming  a  solid  solution 
with  the  metal;  such  for  instance  is  the  action  of  phosphorus  on  iron,  to  be  considered 
later.  This  enlargement  of  the  grain  is  generally  accompanied  by  decreased  ductility 
or  even  by  brittleness. 

The  second  group  of  impurities,  namely  those  foreign  substances,  whether  they 
remain  or  not  uncombined,  which  do  not  form  solid  solutions  with  the  contaminated 
metal  may  usually  be  readily  detected  under  the  microscope  as  they  are  generally 
rejected  to  the  grain  boundaries  during  the  process  of  solidification  (or  afterwards) 
as  shown  in  Figure  7.  These  insoluble  impurities  are  not,  however,  rejected  as  such 
by  the  crystalline  grains,  but  on  the  contrary  unite  mechanically  with  a  small  amount 
of  the  metal  to  form  what  is  known  as  an  "eutectic  alloy,"  that  is  an  alloy  of  lowest 
melting-point,  and  it  is  this  alloy  which  is  expelled  by  the  solidifying  grains.  The 
formation  and  nature  of  eutectic  alloys  will  be  considered  at  greater  length  in  a 
subsequent  lesson. 

It  will  be  apparent  that  those  contaminatir  »  substances  which  fail  to  be  dis- 
solved by  the  metal,  may  form  actual  membram  surrounding  each  grain,  the  mem- 


LESSON   I  — PURE   METALS  7 

branes  being  of  the  nature  of  an  eutectic  alloy.  As  might  be  anticipated,  the  presence 
of  such  membranes,  whether  continuous  or  not,  have  generally  a  very  marked  influence 
upon  the  properties  of  the  metals,  frequently 'decreasing  their  ductility,  weldability, 
electrical  conductivity,  etc.,  and  often  increasing  their  fusibility,  hardness,  etc. 

The  rejection  during  solidification  and  subsequent  cooling  of  those  impurities 
which  fail  to  be  retained  in  solid  solution  by  the  metal,  to  the  grain  boundaries  or 
other  crystallographic  planes,  reveals  the  crystalline  forms  of  the  grains  themselves. 
The  location  of  these  impurities  affords  additional  evidence  of  the  cubic  crystallization 
of  metals.  It  will  be  shown  later  that  the  cubic  crystallization  of  iron  is  in  this  way 
clearly  revealed. 

The  above  remarks  are  of  a  very  general  character  and  refer  more  especially  to 
the  behavior  of  impurities  while  the  metal  solidifies.  In  the  majority  of  cases  no 
further  changes  take  place  in  the  nature  of  the  constituents  as  the  metal  cools  to 
atmospheric  temperature,  i.e.  the  constituents  formed  on  solidification  are  those 
found  in  the  metal  after  complete  and  slow  cooling.  In  some  instances,  however, 
and  notably  in  the  case  of  iron  and  its  usual  impurities,  carbon,  silicon,  phosphorus, 
manganese,  and  sulphur,  some  important  changes  take  place  at  temperatures  con- 
siderably below  the  solidification  point  of  the  metal  which  will  be  duly  described  at 
the  proper  time. 

Influence  of  Thermal  Treatment.  —  The  size  of  the  crystalline  grains  of  pure 
metals  varies  in  different  metals  even  when  cast  and  cooled  under  identical  condi- 
tions. Their  dimension  is  generally  affected  also  by  the  rate  of  cooling  during  solidi- 
fication, and,  therefore,  by  the  size  of  the  casting,  since  a  large  casting  will  cool 
more  slowly  than  a  smaller  one. 

The  common  belief  is  that  the  prolonged  exposure  of  pure  metals  to  a  high  tem- 
perature (annealing)  tends  to  enlarge  the  grains,  the  enlargement  being  the  greater 
the  higher  the  temperature,  the  longer  the  time  of  exposure,  and  the  slower  the 
cooling.  While  such  growth  undoubtedly  takes  place  in  the  case  of  commercial  and, 
therefore,  impure  metals,  at  least  after  straining,  it  is  held  by  some  metallurgists 
that  in  absolutely  pure  metals  the  grain  will  not  grow  on  annealing  even  after 
straining. 

This  view  is  based  upon  a  theory  brilliantly  conceived  by  Ewing  and  Rosenhain  and  supported 
by  the  results  of  skilfully  conducted  experiments.  These  scientists  argue  that  even  so-called  pure 
metals  always  contain  a  certain  amount  of  impurities,  and  that  even  a  very  minute  amount  of  im- 
purity would  suffice  to  form  a  thin  but  practically  continuous  film  of  eutectic  in  the  crystalline 
boundaries.  They  contend  "that  there  is  constant  diffusion  from  the  surface  of  the  crystal  into 
the  eutectic  and  equally  constant  re-deposition  of  metal  upon  the  crystal  from  the  eutectic.  If  there 
are  several  crystals  in  contact  with  the  same  eutectic,  there  will  be,  under  some  conditions,  a  state 
of  dynamic  equilibrium  between  them,  the  amount  dissolved  from  each  being  exactly  counter- 
balanced by  the  amount  deposited  upon  it;  if,  however,  there  is  any  difference  in  their  solution  pres- 
sure in  respect  to  the  eutectic,  then  the  less  soluble  will  grow  at  the  expense  of  the  more  soluble. 
The  metal  constituting  the  eutectic  films  being  much  nearer  its  melting-point  than  the  rest  of  the 
mass,  would  thus  be  favorable  to  comparatively  rapid  diffusion,  but  the  rate  of  such  diffusion,  and, 
consequently,  the  rate  of  growth  of  crystals,  would  be  enormously  increased  by  heating  the  metal 
to  a  temperature  above  the  melting-point  of  the  eutectic  in  question." 

The  theory  proposed  depends  upon  the  existence  of  a  difference  in  the  solubility  of  the  two  crys- 
stal  faces  in  contact  with  the  eutectic  film.  The  only  difference  between  these  two  faces  is,  appar- 
ently, in  the  orientation  of  the  crystalline  elements,  but  this  difference  is  sufficient,  in  the  authors' 
opinion,  to  produce  a  difference  in  their  .rate  of  solution  in  the  eutectic  film,  seeing  that  it  results  in 
such  marked  difference  in  their  solubilit  in  the  etching  acid,  which,  as  is  well  known,  attacks  some 


8  LESSON  I  — PURE  METALS 

grains  much  more  readily  and  deeply  than  neighboring  ones  whose  elements  have  another  orienta- 
tion. To  account  for  the  influence  of  the  orientation  of  the  elements  upon  the  solubility  of  the  crys- 
tals, the  authors  suggest  to  extend  to  alloys  the  electrolytic  theory  of  solution.  "Such  differential 
actions,"  they  say,  "may  most  probably  be  attributed  to  differences  of  electrical  potential  in  the 
surfaces  involved.  If  we  accept  this  view  of  the  matter,  then  the  diffusion  across  films  of  eutectic 
becomes  a  case  of  electrolysis." 

This  theory  explains  why  only  strained  crystals  of  the  metals  examined  will  grow,  while  un- 
strained crystals  show  no  tendency  to  change,  even  at  high  temperatures.  "The  explanation,  on  the 
electrolytic  theory,  is  that  in  the  unstrained  state  the  crystals  are  surrounded  by  practically  con- 
tinuous films  of  eutectic,  and  that  electrolysis  only  becomes  possible  when  severe  distortion  has 
broken  through  these  films  in  places,  allowing  the  actual  crystals  to  come  into  contact;  the  electro- 
lytic circuit  would  then  be  for  each  pair  of  crystals,  from  one  crystal  to  the  other  by  direct  contact 
and  back  through  the  eutectic  film." 

If  the  authors'  conception  be  true,  recrystallization  by  annealing  in  a  perfectly  pure  metal 
would  not  occur  but,  as  they  rightly  say,  it  is  almost  hopeless  to  obtain  a  sample  of  metal  suffi- 
ciently pure  to  prove  or  disprove  the  theory.  They  argue,  however,  that  if  the  growth  of  crystals 
is  due  to  the  presence  of  a  eutectic  film  between  them,  the  absence  of  such  film  would  be  a  barrier 
to  all  such  growth,  and  that  a  weld  between  two  clean-cut  surfaces  should  show  no  growth  of 
crystal  across  the  weld.  This  they  actually  proved  to  be  so  in  the  case  of  lead. 

If  they  are  right,  it  likewise  follows  that  in  metals  contaminated  by  impurities  with  which 
they  form  solid  solutions  there  should  be  no  growth  of  grains  on  annealing,  because  of  the  absence 
of  the  needed  eutectic  film. 

The  remarkable  crystalline  growth  of  very  low  carbon  steel  after  severe  straining 
followed  by  annealing  at  suitable  temperatures,  described  in  another  lesson,  is  a 
striking  instance  of  the  action  of  straining  (cold  .working)  in  promoting  grain  growth 
in  subsequent  annealing. 

Notwithstanding  Ewing's  and  Rosenhain's  strong  argument,  satisfactory  evi- 
dences are  still  lacking  in  support  of  the  contention  that  both  straining  and  the 
presence  of  impurities  are  necessary  conditions  to  grain  growth  on  annealing.  So 
far  as  the  matter  has  been  investigated  it  does  seem  that  the  grain  of  a  pure  metal 
will  not  grow  unless  it  has  been  previously  strained,  but  that  it  must  also  contain 
some  eutectic  forming  impurities  has  not  been  satisfactorily  demonstrated. 

Influence  of  Mechanical  Treatment.  —  Metals  are  frequently  subjected  to  power- 
ful pressure  exerted  by  rolls,  presses,  hammers,  etc.,  with  a  view  of  producing  metallic 
objects  of  desired  shapes.  This  treatment  affects  the  structure  and,  therefore,  the 
properties  of  the  metal.  Roughly  speaking  such  vigorous  kneading  has  a  tendency 
to  reduce  the  size  of  the  final  grains,  either  through  preventing  the  formation  of 
large  grains  or  by  breaking  up  or  distorting  preexisting  grains.  A  smaller  grain  in 
turn  generally  implies  greater  ductility  (provided  it  be  not  distorted)  and  often  greater 
strength.  The  effect  of  work  upon  the  structure  and  properties  of  commercial  iron 
and  steel  will  be  duly  considered  in  these  lessons. 

Examination 

I.     Explain  the  formation  and  nature  of  the  polyhedral  grains  of  which  pure  metals 

are  composed. 
II.     Explain  the  meaning  of  (1)  solid  solutions,  (2)  isomorphous  mixtures,  and  (3) 

mixed  crystals. 

III.     Describe  the  changes  of  structure,  if  any,  produced  in  metals  by  the  presence  of 
small  quantities  of  impurities. 


LESSON   H 

PURE  IRON 

Chemically  pure  iron  is  not  a  commercial  product.  It  can  only  be  obtained  in 
small  quantities  by  carefully  conducted  laboratory  manipulations  when,  even  with 
the  most  refined  care,  it  is  quite  impossible  to  produce  it  absolutely  pure.  Until 
quite  recently  the  purest  commercial  iron  was  of  Swedish  origin  and  contained  as 
much  as  99.8  per  cent  of  iron.  A  commercial  product  known  as  "American  Ingot 
Iron"1  is  now  manufactured  which  the  makers  claim  to  contain  99.94  per  cent  of 


•'   .'. 


••:•    :,-££>> 


Fig.  1.  —  Electrolytic  iron.     Magnified  7.5  diameters.     Slightly  etched. 
(Sherard  Cowper-Coles.) 

iron.  Iron,  or  rather  very  low  carbon  steel,  of  a  high  degree  of  purity  is  also  pro- 
duced at  the  present  time  through  refining  in  electric  furnaces.  Finally  iron  has  been 
obtained  in  relatively  large  quantity,  by  electrolytic  deposition,  of  a  degree  of  purity 
exceeding  99.9  per  cent  iron. 

Microstructure.  —  When  a  sample  of  nearly  pure  iron  is  suitably  prepared  and 
examined  under  the  microscope  some  regions  can  readily  be  found  absolutely  free 

1  The  expression  "ingot"  iron  is  applied  to  iron  containing  very  little  carbon,  obtained  molten 
and,  therefore,  cast,  after  removal  from  the  furnace,  whereas  by  wrought  iron  is  meant  iron  (also 
generally  low  in  carbon)  obtained  pasty  and,  consequently,  always  containing  a  certain  amount  of 
slag.  Barring  the  presence  of  slag  in  wrought  iron,  both  ingot  iron  and  wrought  iron  may  have 
identical  chemical  compositions.  The  expression  ingot  iron  is  seldom  used  in  the  United  States, 
where  iron  obtained  in  a  liquid  state,  and  containing  little  carbon,  is  called  low  or  very  low  carbon 
steel,  or  mild,  very  mild,  extra  mild,  steel,  or  again  soft,  very  soft,  dead  soft  steel. 

1 


LESSON   II  — PURE  IRON 


from  carbon  and  slag  and  exhibiting,  therefore,  the  structure  of  the  pure  metal. 
Such  structure  is  illustrated  in  Figure  1  after  a  slight  etching  of  the  polished  surface. 
It  will  be  noted  that  it  is  similar  to  the  structure  of  pure  gold  and  of  pure  copper 
described  in  the  preceding  lesson :  like  gold  and  copper  and,  indeed,  like  most  metals, 


Fig.  2.  —  Ferrite  grains.     Natural  size. 

Etched  10  minutes  in  nitric  acid  (1  to  10  water). 

(Stead.) 

iron  is  made  up  of  polyhedral  crystalline  grains  (allotrimorphic  crystals).  Upon 
deeper  etching  the  dissimilarity,  as  to  coloration,  of  adjacent  iron  grains  is  clearly 
brought  out  as  shown  in  Figure  2.  As  explained  in  Lesson  I,  this  appearance  is  due 
to  the  fact  that  the  grains  of  iron  are  composed  of  crystalline  elements  having  the 
same  orientation  in  the  same  grain  but  different  ones  in  different  grains. 

Cubic  Crystallization  of  Iron.  —  A  still  deeper  etching  indicates  clearly  the  cubic 
character  of  the  crystallization  of  pure  iron.    This  is  illustrated  diagrammatically  in 


LESSON  II  — PURE  IRON  3 

Figure  3  and  by  means  of  a  photomicrograph  in  Figure  4.  It  will  be  noted  that  the 
etching  pits  are  similarly  oriented  in  the  same  grain  but  that  the  orientation  in  ad- 
jacent grains  differs.  As  seen  in  Figure  3,  the  etching  figures  may  appear  as  triangular 
wedges.  This  occurs  when  the  section  cuts  the  small  cubes  of  a  grain  at  a  certain 


Fig.  3.  —  Etching  pits  in  iron. 
(Desch.) 


Fig.  5.  —  Silicon  steel,  4.5  per 
cent  silicon.  Magnified  60 
diameters.  Part  of  a  single 
grain.  Etched  three  hours  in 
nitric  acid  (1  to  10  water). 
(Stead.) 


Fig.  4.  —  Etching  pits  in  ferrite. 
(J.  F.  Hoyland.) 


Fig.  6.  —  Cubic  crystals  of  iron.  Magnified 
250  diameters.  Obtained  through  the  reduc- 
tion of  ferrous  chloride.  (Osmond.) 


angle,  i.e.  when  it  cuts  obliquely  a  corner  of  each  cube.  This  cubic  structure  is  fur- 
ther illustrated  in  a  remarkable  manner  in  Figure  5,  in  the  case  of  iron  containing 
\Yi  per  cent  of  silicon  after  etching  three  hours  in  dilute  nitric  acid.  The  photo- 
graph shows  a  portion  of  a  single  grain,  hence  the  constancy  of  orientation  noted. 
The  presence  of  so  much  silicon  apparently  favors  the  development  of  a  coarse  cubic 
crystallization. 

Osmond,  through  the  reduction  of  ferrous  chloride  and  the  crystallization  of  the 


LESSON  II  — PURE  IRON 


resulting  metallic  iron,  obtained  perfect  isolated  cubic  crystals  (Fig.  6).  Finally 
almost  perfect  cubes  have  been  separated  by  Stead  from  a  large  granule  of  phos- 
phoretic  iron  (Fig.  7).  Another  indication  of  the  cubic  crystallization  of  iron  is  found 
in  the  occurrence  of  large  crystallites  (Fig.  8),  generally  resembling  pine  trees,  in 
cavities  of  large  castings  of  iron  and  steel,  under  conditions,  therefore,  favorable  to 
the  free  development  of  crystals,  these  crystallites  being  composed  of  small  octa- 
hedra,  a  crystalline  form  of  the  cubic  system.  Finally  it  will  be  shown  later  that 
the  structural  location  of  some  of  the  impurities  generally  present  in  commercial 
iron  affords  further  proof  of  the  cubic  crystallization  of  iron. 

Ferrite.  —  Mineralogical  names  have  been  given  to  the  constituents  of  iron  and 
steel,  and  pure  iron,  or  rather  carbonless  iron,  considered  as  a  microscopical  con- 


Fig.  7.  —  Cubic  crystals  of  phosphoretic  iron. 
Magnified  5  diameters.  Phosphorous  0.75 
per  cent,  carbon  0  per  cent.  (Stead.) 

stituent  has  been  called  "ferrite,"  a  name  suggested  by  Professor  H.  M.  Howe  and 
universally  adopted.1  Pure  iron,  therefore,  is  composed  of  polyhedral  crystalline 
grains  of  ferrite.  It  will  be  seen  in  subsequent  lessons  that  the  ferrite  of  commercial 
grades  of  iron  and  steel  is  not  pure  iron,  but  rather  a  solid  solution  of  iron  holding 
small  amounts  of  silicon,  phosphorous,  and  possibly  other  impurities. 

Allotropy  of  Iron.  —  The  study  of  the  crystallization  of  iron  is  complicated  by 
the  existence  of  several  allotropic  forms  of  that  metal. 

Allotropy  suggests  marked  and  sudden  changes  in  some  of  the  properties  of  a 

1  This  constituent  was  called  "free  iron"  by  Sorby,  who  was  the  first  scientist  to  describe  the 
microscopical  structure  of  iron  and  steel. 

In  the  last  report  (1909)  of  the  Committe  on  the  Uniform  Nomenclature  of  Iron  and  Steel,  ferrite 
is  described  as  follows:  "Alpha  iron  holding  in  solution  in  the  case  of  commercial  grades  of  iron  and 
steel,  small  and  varying  amounts  of  silicon,  manganese,  phosphorus,  and  of  some  other  rarer  ele- 
ments, but  not  more  than  0.05  per  cent  of  carbon,  if  any."  There  is  no  conclusive  evidence,  how- 
ever, of  the  presence  of  carbon  in  ferrite,  nor  is  it  likely  that  iron  retains  any  manganese  in  solid 
solution  when  only  a  small  amount  of  that  impurity  is  present.  (See  Lesson  VI  on  Impurities  in 
Iron  and  Steel.)  Stead  has  suggested  that  when  ferrite  consists  only  of  pure  iron  it  should  be  called 
"ferro-ferrite." 


LESSON   II  — PURE   IROX 


.  g.  —  iron  crystallite  about  half 
natural  size.     (Tschernoff.) 


6  LESSON  II  — PURE  IRON 

substance  occurring  at  certain  critical  temperatures,  without  any  change  of  state  or 
of  chemical  composition.1  Polymorphism,  sometimes  used  as  the  equivalent  of  allo- 
tropy,  refers  more  specifically  to  the  property  of  some  substances  of  crystallizing  in 
more  than  one  form,  while  allotropy  does  not  necessarily  imply  such  property. 

Many  substances  undergo  allotropic  changes.  It  is  a  matter  of  common  knowl- 
edge, for  instance,  that  sulphur  exists  under  two  distinct  conditions,  namely,  as  pris- 
matic sulphur  and  as  octahedral  sulphur,  the  prismatic  form  being  the  one  stable 
above  95.6  deg.  C.  and  the  octahedral,  the  stable  form  below  that  critical  tempera- 
ture. On  heating  octahedral  sulphur  it  begins  to  change  into  the  prismatic  form  at 
the  temperature  of  95.6  deg.,  and,  likewise,  on  cooling,  prismatic  sulphur  begins  to 
pass  to  the  octahedral  form  at  that  temperature.  Many  of  the  physical  properties  of 
sulphur  (crystalline  form,  specific  heat,  heat  of  combustion,  etc.)  undergo  sudden 
changes  as  the  substance  passes  from  one  allotropic  form  to  another. 

At  those  critical  temperatures  which  mark  the  passage  of  one  allotropic  form  into 
another,  spontaneous  evolutions  of  heat  take  place  on  cooling  and  spontaneous  ab- 
sorptions of  heat  on  heating.  These  thermal  disturbances  indicate  a  change  of  in- 
ternal energy  which  when  not  accompanied  by  changes  of  state  or  by  chemical 
changes  are  evidences  of  allotropy.  The  usual  method  of  detecting  the  existence  of 
such  thermal  critical  points  will  be  described  in  another  lesson. 

Osmond's  momentous  discovery  of  the  existence  of  two  thermal  critical  points 
in  pure  iron  proves  the  existence  of  iron  in  at  least  three  allotropic  forms.  The  two 
critical  temperatures  of  pure  iron  correspond  respectively  to  about  875  and  750  deg. 
C.  Above  875  deg.,  iron  exists  in  a  certain  allotropic  condition  known  as  y  (gamma) 
iron.  On  cooling,  when  875  deg.  is  reached  iron  passes  from  the  /  form  to  another 
allotropic  condition  called  ft  (beta)  iron  and  at  750  deg.  ft  iron  in  turn  changes  into  a 
third  allotropic  form,  «  (alpha)  iron,  which  remains  the  stable  form  at  atmospheric 
temperature.  The  reverse  changes  occur  on  heating,  that  is  iron  passes  from  the  a, 
to  the  ft  form  and  then  to  the  7  form  on  passing  through  the  two  critical  tempera- 
tures.2 

As  should  be  expected  the  passage  of  one  allotropic  form  into  another  implies 
corresponding  and,  generally,  sudden  changes  in  many  of  the  physical  properties  of 
iron.  Gamma,  beta,  and  alpha  iron  differ  widely  in  regard  to  many  of  their  physical 
characteristics.  It  is  only  desired  in  this  lesson,  however,  to  inquire  into  the  possible 
differences  of  crystallization  which  may  exist  between  the  three  allotropic  conditions 

1  In  the  Nomenclature  of  Metallography  published  by  the  Iron  and  Steel  Institute  in  1901 
allotropy  is  described  as  "the  capacity  to  undergo  without  change  of  composition  a  change  of  chem- 
ical and  physical  properties." 

Roberts-Austen  defined  allotropy  as  "a  change  of  internal  energy  occurring  in  an  element  at 
a  critical  temperature  unaccompanied  by  a  change  of  state."  In  the  above  definition  the  word 
substance  should  be  used,  however,  in  place  of  element,  for  allotropic  changes  are  not  confined  to 
elements;  chemical  compounds  also,  and  possibly  solid  solutions,  may  undergo  allotropic  trans- 
formations. 

Tiemann  ("Iron  and  Steel")  thus  defines  allotropy  "a  change  in  the  properties  of  an  element 
without  change  of  state.  It  is  habitually  accompanied  by  a  change  of  internal  energy.  It  is  due  in 
some,  and  perhaps  in  all,  cases  to  a  change  in  the  number  or  in  the  arrangement  of  the  atoms  of 
the  molecule  (Howe).  Allotropic  varieties  are  sometimes  termed  'isomerides.'" 

2  Le  Chatelier  and  some  other  writers,  while  admitting  the  existence  of  a  lower  critical  point  in 
pure  iron,  doubt  that  it  indicates  an  allotropic  change,  being  consequently  inclined  to  deny  the 
existence  of  beta  iron.    This  opinion  will  be  considered  and  the  allotropy  of  iron  treated  at  greater 
length  in  another  lesson. 


LESSON   II  — PURE   IRON  7 

of  iron,  postponing  until  a  later  lesson  a  description  of  the  modification  of  the  other 
properties. 

Osmond  and,  later,  Osmond  and  Cartaud  have  carefully  investigated  the  difficult 
problem  of  the  crystallization  of  the  different  allotropic  forms  of  iron.  Their  con- 
clusions were  (1)  that  the  three  allotropic  forms  of  iron  crystallize  in  the  cubic  sys- 
tem,1 (2)  that  octahedra  are  the  prevailing  crystalline  forms  of  gamma  iron,  (3)  that 
the  cube  is  the  prevailing  form  of  beta  and  of  alpha  iron,  (4)  that  beta  and  alpha  iron 
are  capable  of  forming  isomorphous  mixtures  (solid  solutions),  (5)  that  gamma  iron 
docs  not  form  isomorphous  mixtures  with  beta  or  alpha  iron.  . 

We  also  have  the  statement  of  Osmond  that  the  transformation  of  gamma  iron 
into  beta  iron  appears  to  include  a  change  in  the  planes  of  symmetry,  at  least  in  car- 
burized  iron. 

Again  it  has  been  shown  by  Osmond  and  confirmed  by  other  investigators  that 
the  occurrence  of  twinnings  is  frequent  in  gamma  iron,  while  beta  and  alpha  iron  are 
free  from  it.  By  twinning  is  meant  the  grouping  of  two  or  more  crystals  or  parts  of 
crystals  in  such  a  way  that  they  are  symmetrical  to  each  other  with  respect  to  a 
plane  between  them  (the  twinning  plane)  which  plane,  however,  is  not  a  plane  of 
symmetry  for  either  crystal. 

Twinnings  are  also  produced  through  pressure  or  other  stress  tending  to  strain 
the  crystals  especially  when  the  straining  is  followed  by  annealing.  In  Figures  9,  10, 
and  11  are  shown  twinnings  respectively  in  marble  (due  to  pressure),  in  brass,  and  in 
gamma  iron.  The  straight  parallel  lines  and  bands  running  through  the  polyhedral 
grains  indicate  the  twinning  planes. 

From  these  conclusions  which  have  been  confirmed  by  the  results  obtained  by 
other  investigators,  it  follows  that  the  allotropy  of  iron  could  not  be  proved  by  its 
crystallography,  since  the  thermal  critical  points  are  not  accompanied  by  changes  in 
the  crystalline  form  of  iron.  While,  however,  in  the  instances  of  allotropy  which  have 
been  noted  and  studied  allotropic  changes  are  generally  accompanied  by  changes  of 
crystalline  forms,  it  does  not  by  any  means  follow  that  any  allotropic  transformation 
must  necessarily  imply  a  crystalline  change. 

Bearing  in  mind  the  existence  of  three  allotropic  conditions  of  iron,  let  us  follow 
in  our  imagination  the  crystallization  of  iron  during  solidification  and  its  subsequent 
cooling  to  atmospheric  temperature.  On  solidifying,  polyhedra  or  allotrimorphic 
crystals  of  gamma  iron  are  formed,  which  according  to  Osmond  are  chiefly  made  up 
of  octahedral  crystals.  Were  it  possible  to  examine  a  section  of  the  solidified  metal 
at  such  a  high  temperature  the  usual  polygonal  network  structure  characteristic  of 
pure  metals  would  be  revealed.  Upon  further  cooling  below  the  solidification  point, 
no  change  of  crystalline  form  should  take  place  until  the  first  critical  temperature 
(875  deg.  C.)  is  reached  when  the  iron  changes  from  the  gamma  to  the  beta  condition. 

Does  this  allotropic  change  affect  the  preexisting  crystallization  of  gamma  iron 
or  does  it  consist  merely  in  a  transformation  in  situ  of  each  crystalline  grain  of  gamma 
iron  into  a  grain  of  beta  iron,  retaining  the  original  external  form  of  the  gamma 
grain,  and  leaving  undisturbed,  therefore,  the  polygonal  structure  observed  under  the 
microscope?  It  may  reasonably  be  supposed  that  the  allotropic  transformation  takes 
place  without  affecting  the  external  form  of  the  crystalline  grains,  but  in  view  of 

1  In  Le  Chatelier's  opinion  there  is  no  proof  of  the  cubic  form  of  gamma  iron.  He  thinks  that 
the  facts  observed  are  contrary  to  that  hypothesis  and  that  it  is  more  probable  that  gamma  iron  is 
rhomboh'edral  or  orthorhombic. 


8 


LESSON  II  — PURE  IRON 


Osmond's  statement  that  the  octahedron  is  the  prevailing  crystalline  form  of  gamma 
iron  and  bearing  in  mind  that  the  small  crystals  revealed  by  suitable  etching  of  pure 
iron  are  generally  cubic,  we  naturally  infer  that  the  octahedral  character  of  each 
grain  of  gamma  iron  has  been  obliterated,  the  small  octahedral  elements  of  gamma 


Fig.  9.  —  Twinnings  in  marble  (caused  by 

pressure).     Magnified  about  5  diameters. 

(Bayley.) 


Fig.  11.  —  Twinnings  in  gamma  iron. 
Magnified  200  diameters.     (Osmond.) 


Fig.  10.  —  Twinnings  in  brass.     Magnified  100  diameters.     (Law.) 

iron  having  been  replaced  by  small  cubic  elements  of  beta  (and  later  of  alpha)  iron. 
These  conclusions,  however,  should  be  accepted  with  reserve,  as  we  lack  evidences  of 
a  very  conclusive  character.  It  has  been  shown  that  the  change  of  gamma  iron  into 
beta  iron  is  accompanied  by  an  abrupt  expansion  of  the  cooling  metal  which  expan- 
sion is  followed  by  the  normal  contraction  of  cooling  substances.  We  infer  from 


LESSON   II  — PURE   IRON 


9 


this  that,  momentarily  at  least,  each  grain  of  beta  iron  occupies  more  space  than  its 
gamma  iron  progenitor. 

At  750  deg.  C.  or  thereabout,  the  iron  passes  from  the  beta  to  the  third  or  alpha 
form.  We  may  here  ask  the  same  questions  as  to  the  probable  effect  of  this  change 
upon  (1)  the  outward  form  of  each  beta  grain  and  (2)  upon  the  internal  crystalline 
structure  of  each  grain.  Since  both  beta  and  alpha  iron  crystallize  in  the  cubic  sys- 
tem, the  cube  being  the  prevailing  crystalline  form  of  both,  and  since,  according  to 
Osmond,  they  are  isomorphous,  that  is  capable  of  forming  solid  solutions,  it  seems 
probable  that  the  change  of  beta  to  alpha  iron  affects  neither  the  external  form  nor 


Fig.  12.  —  Polished  sample  of  iron,  etched,  then 
reheated  in  hydrogen  to  950  deg.  C.  and  slowly 
cooled.  Magnified  600  diameters.  (Kroll.) 


the  internal  crystalline  arrangement  of  the  beta  grains;  in  other  words  that  each 
small  cubic  element  of  beta  iron  is  converted  bodily  (although  probably  gradually 
and  not  abruptly)  into  a  cubic  element  of  alpha  iron. 

If  the  above  represents  the  real  mechanism  of  the  allotropic  changes,  it  will  be 
evident  that  the  polyhedral  grains  revealed  through  the  etching  of  polished  sections 
of  pure  iron  were  formed  during  solidification  and  consisted  originally  of  gamma 
iron,  these  grains  having  retained  their  external  shape  while  undergoing  allotropic 
transformation,  but  having  probably  undergone  at  the  upper  critical  point  an  internal 
change,  the  octahedral  form  of  the  crystalline  elements  having  been  replaced  by  the 
cubic  form. 

Some  experimental  evidences,  however,  have  recently  been  brought  forward 
which  would  lead  to  somewhat  different  conclusions.  Kroll  in  a  valuable  and  sug- 
gestive contribution  expresses  his  belief  that  he  has  been  able  to  develop  in  a  sample 
of  nearly  pure  iron  three  distinct  polygonal  networks,  corresponding  respectively  to 
the  crystallization  of  gamma,  beta,  and  alpha  iron.  This  interesting  composite  struc- 
ture is  illustrated  in  Figure  12.  It  was  obtained  in  heating  a  polished  section  of  pure 


10  LESSON  II  — PURE  IRON 

iron  in  a  current  of  hydrogen  gas,  when  it  was  found  that  the  three  polygonal  struc- 
tures were  revealed.  If  the  meaning  of  this  structure  is  correctly  interpreted  by 
Kroll,  it,  of  course,  points  to  marked  crystallographic  changes  closely  related  to  the 
thermal  points  of  Osmond.  As  Professor  Le  Chatelier  rightly  says,  however,  the 
accuracy  of  Mr.  KrolPs  conclusions  might  be  questioned,  because  changes  of  struc- 
ture must  take  place  on  cooling  as  well  as  on  heating  so  that  five  and  not  three  net- 
work structures  should  be  observed. 

Rosenhain  and  Humfrey  by  straining  a  bar  of  iron  heated  to  a  high  temperature 
at  the  center  (the  temperature,  therefore,  decreasing  towards  both  ends)  have  made 
evident  the  existence  of  three  distinct  kinds  of  distortions  with  sharp  lines  of  de- 
marcation between  them,  corresponding  in  all  probability  to  the  distortion  respect- 
ively of  gamma,  beta,  and  alpha  iron.  The  estimation  of  the  temperature  of  different 
portions  of  the  bar  by  means  of  fusible  salts  appears  to  sustain  the  authors'  con- 
tention thus  furnishing  an  additional  support,  and  a  substantial  one,  to  Osmond's 
brilliant  theory  of  the  allotropy  of  iron. 

Nevertheless  information  of  a  more  positive  and  concordant  nature  is  still  needed 
to  settle  to  the  satisfaction  of  all  the  question  of  the  relation  between  the  thermal 
critical  points  of  pure  iron  and  possible  crystallographic  changes. 

While  the  allotropy  of  iron  is  of  little  industrial  importance  in  the  case  of  iron 
and  of  low  carbon  steel  it  becomes,  on  the  contrary,  of  very  great  moment  in  high 
carbon  steel,  for  it  is  probably  the  cause  of  that  invaluable  property  possessed  by 
such  steels  of  becoming  intensely  hard  upon  rapid  cooling  from  a  sufficiently  high 
temperature.  The  question  of  the  hardening  of  steel  will  be  duly  considered  in  these 
lessons. 

Influence  of  Impurities.  —  Commercial  iron  is  always  contaminated  by  the 
presence  of  at  least  five  elements,  namely,  manganese,  silicon,  phosphorus,  sulphur, 
and  carbon,  generally  referred  to,  although  often  wrongly,  as  impurities.  The  im- 
portant question  of  the  influence  of  these  substances  upon  the  structure,  and,  there- 
fore, upon  the  properties,  of  iron  and  steel  will  be  fully  considered  in  another  lesson, 
when  it  will  be  shown  that  these  elements  form  definite  compounds  with  iron,  FeSi, 
Fe3C,  Fe3P,  or  with  each  other,  Mn3C,  MnS,  and  that  some  of  these  compounds, 
FeSi,  Fe3P,  are  retained  by  the  iron  in  solid  solution,  while  others,  Fe3C,  Mn3C,  MnS, 
are  rejected  to  the  boundaries  of  the  crystalline  grains  or  along  other  crystallographic 
planes,  the  former  two  giving  rise  to  the  formation  of  eutectoid  mixtures.  This 
behavior  of  the  impurities  of  iron  and  steel  conforms  with  the  general  behavior  of 
impurities  described  in  Lesson  I. 

Influence  of  Heat  Treatment.  —  The  size  of  the  grains  of  iron  is  affected,  like 
that  of  other  metals,  by  the  temperature  from  which  it  cools,  the  length  of  time  it  is 
kept  at  that  temperature,  the  rate  of  cooling,  etc.,  in  other  words  by  what  is  known 
as  its  heat  or  thermal  treatment.  Generally  speaking  it  may  be  said  that  the  higher 
the  temperature  the  larger  the  grains  and  also  that  the  slower  the  cooling  the  larger 
the  grains.  These  results  might  have  been  anticipated  if  it  be  considered  that  slow 
cooling  from  a  high  temperature  is  a  condition  favorable  to  the  formation  and  growth 
of  crystals.  As  stated  in  Lesson  I,  however,  it  is  not  certain  that  pure  metals  under- 
go any  crystalline  growth  on  reheating  (annealing)  unless  they  have  previously  been 
strained  and,  indeed,  unless  they  contain  also  a  trace  at  least  of  impurities.  For 
similar  reasons  we  may  doubt  the  existence  of  any  crystalline  growth  in  annealing 
chemically  pure  iron  or  indeed  impure  iron  unless  it  has  been  previously  strained. 


LESSON  II  — PURE  IRON 


11 


The  influence  of  heat  treatment  upon  the  structure  and  physical  properties  of 
commercial  irons  and  steels  will  be  dealt  with  at  length  in  these  lessons. 

Influence  of  Mechanical  Treatment.  —  Like  that  of  other  metals  the  structure  of 
iron  is  affected  by  mechanical  work.  Undisturbed  cooling  being  a  necessary  condi- 
tion to  the  free  development  of  crystals,  it  will  be  evident  that  if  the  metal  be  vigor- 
ously worked,  that  is  subjected  to  powerful  mechanical  pressure,  while  cooling  from 
a  high  temperature,  the  formation  of  crystalline  grains  will  be  greatly  hindered  or 
preexisting  crystals  broken  or  distorted.  The  important  influence  of  work  upon  the 
structure  (and  therefore  upon  the  properties)  of  iron  and  steel  will  be  duly  considered 
in  these  lessons. 

Straining  of  Iron.  —  Slip  Bands.  —  Ewing  and,  later,  Ewmg  -and  Rosenhain 
through  some  skilfully  conducted  experiments  and  convincing  reasoning  have  re- 
vealed the  character  of  the  strain  produced  in  a  pure  metal  by  the  action  of  a  stress 


Fig.  13.  —  Slip  bands  in  Swedish  iron  strained 

by  tension.     Magnified  400  diameters. 

(Ewing  and  Rosenhain.) 


which  may  eventually  cause  its  rupture.  This  will  be  briefly  described  here  in  the 
case  of  pure  iron. 

Polished  sheets  of  metals  were  strained  very  gradually  while  being  examined 
under  the  microscope.  When  the  yield  point  is  reached,  i.e.  when  plastic  deforma- 
tion begins  to  occur,  black  lines  are  seen  to  cross  the  crystalline  grains  of  which 
the  metal  is  made  up.  These  lines  are  generally  quite  straight  and  are  parallel  in  the 
same  grains  but  have  different  directions  in  different  grains.  Figure  13  shows  the 
appearance  under  vertical  light  of  Swedish  iron,  strained  by  tension,  magnified  400 
diameters. 

As  the  strain  increases  other  systems  of  lines,  inclined  to  the  first,  make  their 
appearance  and  eventually  two,  three,  and  even  four  systems  of  intersecting  lines 
may  be  seen  in  each  grain. 

These  lines  are  not  cracks  but  steps  in  the  surface,  which  steps  are  due  to  slips 
along  the  cleavage  or  gliding  planes  of  the  crystals.  In  Figure  14,  AB  represents  the 


12  LESSON  II  — PURE  IRON 

polished  surface  of  two  grains,  C  the  junction  line  between  these  two  grains.    The 
pull  takes  place  in  the  direction  of  the  arrows. 

Yielding  occurs  by  finite  amounts  of  slips  at  a  limited  number  of  places,  a,  b,  c, 
d,  e,  resulting  in  short  portions  of  inclined  cleavage  or  sliding  surface  appearing  black 
under  the  microscope,  because  they  do  not  send  back  any  light  into  the  tube.  By 
oblique  light  some  of  these  slip  bands  appear  black  while  others  are  bright.  When 
the  surface  is  rotated  some  of  the  bands  which  were  black  become  bright  and  vice 
versa  owing  to  the  change  of  their  position  in  regard  to  the  incidence  of  the  light, 


Before  straining. 
a,         o      ~_       *• 


After  straining 

Fig.  14.  —  Diagram  illustrating  the  effect  of  strain 

upon  the  structure  of  metals  and  alloys. 

(Ewing  and  Rosenhain.) 

which  is  conclusive  proof  that  the  black  lines  are  not  cracks,  but  inclined  surfaces  as 
described  above. 

It  is  seen  then  that,  contrary  to  the  general  belief,  metals  remain  crystalline 
after  the  severest  strain,  and  that  the  flow  or  plastic  strain  in  metals  is  not  a  homo- 
geneous shear  such  as  occurs  in  the  flow  of  viscous  fluids,  but  is  the  result  of  a  limited 
number  of  separate  slips,  the  crystalline  elements  themselves  undergoing  no  deforma- 
tion. 

Examination 

I.  Describe  the  microstructure  of  pure  iron  as  revealed  by  the  microscope  (1) 
after  a  slight  etching  of  the  polished  surface,  (2)  after  a  deeper  etching,  and 
(3)  after  a  still  deeper  etching. 

II.     Give  some  evidences  of  the  cubic  crystallization  of  iron. 
III.     Describe  the  allotropy  of  iron. 


LESSON  III 

WROUGHT  IRON 

Wrought  iron  is  the  name  given  to  commercial  iron  free  enough  Trom  carbon  and 
other  impurities  to  be  malleable  when  such  metal  is  manufactured  through  the  re- 
duction of  iron  ores  or  the  refining  of  cast  iron  at  a  temperature  so  low  that  it  is 
obtained  in  a  pasty  condition  and,  therefore,  mechanically  mixed  with  a  considerable 
amount  of  the  slag  formed  during  the  operation. 

When  the  refining  treatment  is  conducted  at  a  temperature  sufficiently  high  to 
deliver  the  resulting  products  in  a  molten  condition,  the  refined  metal  which  is  then 
free  from  slag  is  called  steel.  Wrought  iron  and  steel  may  otherwise  have  identical 
chemical  composition,  although  usually  steel  contains  more  manganese  and  less 
silicon  than  wrought  iron,  often  more  carbon  and  less  phosphorus. 

Commercial  iron  which  is  not  malleable  is  called  cast  (pig)  iron.  The  modern 
method  of  producing  wrought  iron  consists  in  refining  cast  iron  in  a  non-regenera- 
tive reverberatory  furnace  (the  puddling  furnace),  while  the  refining  of  cast  iron  for 
the  production  of  steel  is  conducted  (1)  in  the  Bessemer  converter  or  (2)  in  a  regen- 
erative reverberatory  furnace  (the  Siemens  open-hearth  furnace).  Cast  (pig)  iron  is 
the  result  of  smelting  iron  ore  in  blast-furnaces. 

Chemical  Composition.  —  Wrought  iron  contains,  besides  an  appreciable  amount 
of  slag,  a  small  proportion  of  carbon  and  small  quantities  of  the  usual  impurities, 
manganese,  silicon,  phosphorus,  and  sulphur. 

Microstructure  of  Longitudinal  Section.  —  Upon  being  withdrawn  from  the 
puddling  furnace,  the  white  hot,  pasty  balls  of  wrought  iron  are  subjected  to  vigorous 
forging  or  squeezing,  thus  expelling  a  large  amount  of  slag  and  firmly  welding  to- 
gether the  particles  of  iron.  Through  additional  heating  and  forging  or  rolling  the 
metallic  mass  is  converted  into  such  elongated  shapes  as  blooms,  billets,  bars,  etc. 
These  operations  so  affect  the  structure  as  to  impart  unlike  appearances  to  sections 
cut  longitudinally,  i.e.  in  the  direction  of  forging  or  rolling,  and  sections  cut  trans- 
versally,  i.e.  at  right  angles  to  that  direction.  The  microstructure  of  the  longitudinal 
section  of  a  wrought-iron  bar  is  shown  in  Figures  1  and  2.  From  our  knowledge  of 
the  chemical  composition  of  wrought  iron  we  should  be  able  to  anticipate  its  micro- 
structure.  The  ground  mass  or  matrix  of  the  metal  consists  of  polyhedral  crystalline 
grains  of  iron,  that  is  of  ferrite,  similar  in  every  respect  to  the  crystalline  grains  of 
pure  iron  and  of  pure  metals  in  general  described  in  Lessons  I  and  II.  The  ferrite  of 
wrought  iron,  however,  as  explained  in  Lesson  II,  is  not  pure  iron  but  rather  a  solid 
solution  of  iron  in  which  are  dissolved  small  quantities  of  silicon,  phosphorus,  and  other 
minor  impurities.  This  true  character  of  commercial  ferrite  is  too  often  lost  sight  of 
and  the  constituent  considered  as  pure  iron.  The  difference  in  coloration  between 
adjacent  grains  of  this  commercial  ferrite  should  be  noted,  and  will  of  course  be  readily 
understood  in  view  of  the  explanation  given  in  Lesson  I  to  account  for  this  phenom- 

1 


2  LESSON   III  — WROUGHT   IRON 

enon.  Many  irregular  black  lines,  varying  much  in  thickness  and  length,  but  all 
running  in  the  same  direction  are  clearly  seen.  These  lines  indicate  the  location  of 
the  slag  which  has  assumed  the  shape  of  fibers  or  streaks  running  in  the  direction  of 
the  rolling  or  forging,  thus  imparting  a  fibrous  appearance  to  the  metal. 

The  presence  of  a  small  amount  of  carbon  in  wrought  iron  results  in  the  occurrence 
of  a  new  constituent  in  the  shape  of  small  dark  particles  located  between  some  of  the 
grains.  Under  low  magnification  these  carbon-holding  particles  are  not  readily  dis- 
tinguishable from  slag  particles  and  as  this  carburized  constituent  is  not  a  very  im- 
portant one  in  the  case  of  wrought  iron  it  seems  advisable  to  postpone  its  description. 


Fig.  1 .  —  Wrought  iron.     Longitudinal 

section.     Magnification  not  stated.  Fig.  2.  —  Wrought  iron.     Longitudinal  section. 

(Longmuir.)  Magnified  100  diameters.     (Boynton.) 

Summing  up,  wrought  iron  consists  essentially  of  a  mass  of  ferrite  containing  many 
elongated  particles  of  slag. 

Microstructure  of  Transverse  Section.  —  The  microstructure  of  the  transverse 
section  of  a  wrought-iron  bar  is  illustrated  in  Figures  3  and  4.  Like  the  structure  of 
the  longitudinal  section,  it  consists  of  a  polygonal  network,  indicating  that  the  metal 
is  made  up  of  polyhedral  crystalline  grains  of  ferrite.  The  slag,  however,  which  in 
the  longitudinal  section  occurred  as  fibers  running  in  a  direction  parallel  to  the  rolling 
or  forging,  here  assumes  the  shape  of  irregular,  dark  areas,  corresponding  to  the  cross 
sections  of  the  slag  fibers.  It  will  be  noted  that  in  both  the  longitudinal  and  trans- 
verse sections  the  ferrite  grains  are  equiaxed,  that  is,  they  show  no  sign  of  having 
been  elongated  in  the  direction  of  the  rolling.  It  was  thought  for  many  years  that 
wrought  iron  actually  had  a  fibrous  structure  and,  indeed,  the  number  of  persons 
still  holding  this  view  is  surprisingly  large.  Many  valuable  properties  were  attrib- 
uted to  puddled  iron  on  account  of  its  "fibrous  structure"  which  were  denied  to 
steel  because  of  its  "crystalline  structure."  The  microscope  has  summarily  dis- 
posed of  this  erroneous  belief  in  showing  that  the  ferrite  constituting  the  bulk  of 
wrought  iron  is  in  no  way  different  from  the  ferrite  forming  the  bulk  of  low  carbon 
steel.  Both  are  equally  crystalline. 


LESSON  III  — WROUGHT  IRON  3 

Chemical  Composition  of  Slag.  —  The  essential  chemical  constituents  of  the  slag 
produced  in  the  puddling  furnace  and  retained  in  part  by  the  iron  are  iron  oxides, 
both  ferric  (Fe2O3)  and  ferrous  (FeO),  oxide  of  manganese  (MnO),  silica  (Si02),  and 
phosphoric  acid  (P2O5).  Of  these  the  oxides  of  iron  and  manganese  are  basic  in  their 
chemical  affinity  while  silica  and  phosphoric  acid  are  acid.  These  bases  and  acids 
combine  with  each  other  to  form  neutral  compounds:  silicates  and  phosphates  of 
iron  and  manganese. 

Microstructure  of  Slag.  —  It  will  be  seen  in  Figures  1  and  2  that  the  slag  fibers 
are  really  made  up  of  at  least  two  constituents,  a  dark  and  a  lighter  one,  the  light 


Fig.  3.  —  Wrought  iron.     Transverse 

section.     Magnification  not  stated. 

(Longmuir.) 


Fig.  4.  —  Wrought  iron.     Transverse  section. 
Magnification  not  stated.     (Guillet.) 


constituent  moreover  often  occurring  in  the  form  of  small  rounded  areas.  This 
structure  of  the  slag  is  shown  more  clearly  and  on  a  larger  scale  in  Figure  5.  We 
are  naturally  led  to  speculate  as  to  the  nature  of  these  two  distinct  constituents  of 
the  slag,  and  in  view  of  our  knowledge  of  the  chemical  composition  of  slag  as  stated 
in  the  preceding  paragraph  we  are  tempted  to  conclude  that  one  of  the  constituents 
is  a  silicate  of  iron  and  manganese  while  the  other  is  a  phosphate  of  the  same  bases. 
The  accuracy  of  this  deduction,  however,  remains  to  be  proven. 

According  to  Matweieff  the  rounded  light  areas  consist  of  iron  oxide  mixed  or 
not  with  manganese  oxide,  and  the  darker  background  of  silicate  of  iron  and  man- 
ganese. 

Matweieff  recommends  the  following  method  to  distinguish  between  the  different  constituents 
of  slag.  The  polished  sample  placed  in  a  tube  is  heated  and  treated  by  a  current  of  pure  hydrogen 
which  causes  the  reduction  of  the  metallic  oxides  while  the  silicates  are  unaffected.  To  detect  the 
presence  of  ferrous  oxide  (FeO)  the  sample  heated  to  a  red  heat  is  acted  upon  by  steam,  a  treatment 
resulting  in  oxidizing  the  ferrous  oxide  into  magnetic  oxide  (Fe3O4),  while  the  silicates  again  remain 
unaltered.  To  detect  the  presence  of  manganese  in  the  particles  of  oxides  revealed  by  the  hydrogen 
treatment  the  previously  treated  sample  is  repolished  and  etched  with  a  dilute  solution  of  ferric 
chloride  in  alcohol:  if  the  white  metallic  grains  resulting  from  the  hydrogen  treatment  are  colored 
darker  than  the  surrounding  iron  they  contain  some  manganese.  Finally  to  detect  the  presence  of 
iron  and  manganese  sulphide  the  polished  sample  is  etched  with  a  dilute  solution  of  tartaric  acid 
which  colors  sulphide  of  manganese  lightly  and  iron  sulphide  decidedly. 


4  LESSON  III  — WROUGHT  IRON 

Rosenhain  considers  it  probable  that  the  two  distinct  constituents  of  wrought- 
iron  slag  are  two  different  silicates  or,  possibly,  oxides  of  iron. 

It  is  seen  that  writers  generally  ignore  the  presence  of  phosphoric  acid  in  the  slag 
from  the  puddling  furnace,  and  still  it  generally  contains  from  3  to  5  per  cent  of  it, 
and  occasionally  considerably  more.  If  we  assume  that  this  phosphoric  acid  forms 
with  iron  a  phosphate  of  the  formula  3FeO.P2O6,  a  simple  calculation,  according  to 
atomic  weights  of  the  elements  involved,  will  show  that  the  presence  of  5  per  cent  of 
phosphoric  acid  would  mean  the  formation  of  over  12.50  per  cent  of  this  phosphate 
of  iron.  It  is  hardly  to  be  supposed  that  this  phosphate  is  absorbed  by  some  other 


Fig.  5.  —  Particle  of  slag  in  wrought  iron. 
Magnified  200  diameters.     (Guillet.) 

constituent  of  the  slag.     On  the  contrary  it  seems  highly  probable  that  it  must  be 
present  as  a  distinct  constituent. 

Influence  of  Thermal  and  Mechanical  Treatments.  —  The  dimensions  of  the  fer- 
rite  grains  of  wrought  iron  are  affected  by  the  treatments,  both  thermal  and  mechan- 
ical, received  by  the  metal.  The  effect  of  these  treatments  upon  the  structure  of 
iron  and  steel  will  be  considered  in  another  lesson. 


Experiments 

A  small  piece  of  wrought-iron  bar  should  be  procured  measuring  preferably  Yi  in. 
square  or  round  and  %  in.  long.  This  piece  should  be  sawed  in  two  longitudinally, 
conveniently  with  a  hack-saw  (preferably  a  power  hack-saw),  and  one  of  the  freshly 
cut  surfaces  prepared  for  microscopical  examination. 

The  various  methods  which  have  been  used  or  recommended  for  the  polishing  of 
iron  and  steel  specimens  preliminary  to  their  microscopical  examination  will  be 
found  duly  described  in  an  appendix  to  these  lessons.  In  connection  with  the  experi- 
ments described  in  this  book  only  those  methods  will  be  mentioned  which  in  the 
author's  opinion  have  been  found  most  satisfactory. 

Polishing  by  Hand.  —  The  sharp  edges  of  the  sample  should  be  filed  or  ground 
in  order  to  avoid  tearing  the  polishing  cloths  in  the  following  operations.  The  sur- 


LESSON   III  — WROUGHT   IRON  5 

face  to  be  prepared  should  be  filed  first  with  a  coarse  and  then  with  a  smooth  file  so 
as  to  obtain  a  perfectly  flat,  smooth  surface.  This  filing  can  be  advantageously  re- 
placed by  grinding  on  a  fine  emery-wheel.  It  is  recommended  that  both  filing  or 
grinding  be  conducted  with  a  very  gentle  pressure.  In  case  the  polishing  is  to  be 
done  by  hand  the  hand  polishing  outfit  described  at  the  beginning  of  these  lessons  is 
recommended. 

A  small  amount  of  No.  80  emery  powder1  mixed  with  sufficient  water  to  form  a 
thick  paste  should  be  placed  on  one  of  the  polishing  blocks  covered  with  cotton 
cloth.  This  paste  should  be  spread  over  the  block,  conveniently  by  means  of  a 
spatula,  and  with  the  addition  of  a  little  more  water  if  necessary.  The  sample  of 
metal  should  now  be  rubbed  back  and  forth  over  this  block,  "Being  careful  to  rub 
always  in  the  same  direction  until  the  marks  left  by  the  file  or  emery-wheel  have  all 
been  removed  and  replaced  by  finer  markings  due  to  the  action  of  the  emery  powder. 
After  this  treatment  the  sample  should  be  carefully  washed,  as  well  as  the  fingers  of 
the  operator,  preferably  in  running  water,  and  the  sample  rubbed  over  the  second 
polishing  block  covered  with  cotton  cloth  and  a  little  flour  emery  and  water,  pre- 
cisely as  before.  On  passing  from  the  first  to  the  second  polishing  block,  the  sample 
should  be  turned  at  right  angles  and  kept  in  that  position,  in  order  that  the  new 
marks  may  be  perpendicular  to  the  old  ones,  and  the  polishing  should  be  continued 
until  the  marks  left  by  the  coarse  emery  have  been  entirely  effaced  and  replaced  by 
finer  ones.  The  sample  after  being  carefully  washed  is  ready  for  the  next  block. 
Some  of  the  tripoli  powder  should  be  spread,  with  the  addition  of  water,  over  one  of 
the  blocks  covered  with  broadcloth,  and  the  sample  polished  upon  this  block  until 
the  markings  left  by  the  previous  polishing  have  been  completely  removed.  After 
careful  washing  the  sample  should  now  be  rubbed  over  the  last  polishing  block,  cov- 
ered with  broadcloth,  rouge,2  and  water,  holding  as  usual  the  sample  so  as  to  rub  it 
at  right  angles  to  the  markings  left  by  the  tripoli.  After  these  markings  have  been 
removed  the  sample  should  have  a  very  bright  surface  and  be  free  from  even  micro- 
scopical scratches.  At  this  stage  a  magnifying-glass  is  very  useful  for  inspecting 
the  specimen  in  order  to  ascertain  whether  it  is  ready  for  the  etching  treatment.  For 
this  purpose  the  vertical  magnifier  described  and  illustrated  under  "Apparatus"  at 
the  beginning  of  these  lessons  will  be  found  very  satisfactory. 

On  examining  the  polished  sample  of  wrought  iron  with  the  naked  eye  many 
small,  elongated  cavities  will  be  detected  which  will  be  more  apparent  still  if  viewed 
through  a  magnifying-glass.  These  marks  correspond  to  the  location  of  the  slag 
fibers  and  will  be  readily  distinguished  from  scratches. 

The  specimen  should  now  be  carefully  washed  and  dried  with  a  soft  cloth,  pref- 
erably a  fine  piece  of  old  linen.  Where  an  air-blast  is  at  hand,  as  is  generally  the 
case  in  chemical  laboratories,  it  is  advisable  to  dry  the  specimen  with  this  blast  (a 
hot  blast  is  more  effective  than  a  cold  one)  instead  of  rubbing  it  with  a  cloth.  The 
sample  may  then  be  passed  gently  once  or  twice  on  a  piece  of  chamois  leather 
stretched  over  a  smooth  piece  of  wood  and  carefully  protected  from  dust  when  not  in 
use,  when  it  will  be  ready  for  the  next  or  etching  operation.  When  polishing,  the 
sample  should  be  pressed  lightly  upon  the  blocks  and  great  care  taken  not  to  carry 
any  coarse  powder  over  a  polishing  block  upon  which  a  finer  powder  is  used  as  the 
presence  of  but  a  few  coarser  grains  will  greatly  lengthen  the  operation.  It  is  of 

1  The  polishing  powders  should  be  of  the  very  best  quality  obtainable. 

2  It  is  essential  to  use  the  best  commercial  grade  of  jeweler's  rouge. 


6  LESSON   III  — WROUGHT  IRON 

much  importance,  therefore,  to  keep  all  the  blocks  carefully  covered  when  not  in 
use  as  well  as  the  bottles  containing  the  powders. 

Polishing  by  Power.  —  Polishing  by  hand  is  at  best  a  tedious  and  laborious 
operation  and  whenever  possible  it  is  highly  advisable  to  replace  it  by  the  use  of  a 
power  polishing  machine.  Very  satisfactory  and  effective  polishing  machines  and 
polishing  motors  are  illustrated  and  described  in  the  introductory  chapter  on  ap- 
paratus. 

When  using  these  polishing  machines  or  polishing  motors  the  manipulations  are 
as  follows: 

The  metal  surface  to  be  prepared  is  pressed  lightly  upon  the  emery-wheel  until  a 
perfectly  flat  surface  is  obtained,  when  it  should  be  washed  with  the  usual  precau- 
tions and  pressed  upon  the  cloth-covered  cast-iron  disk  placed  next  to  the  emery- 
wheel  and  upon  which  flour  emery  and  water  have  been  applied.  Care  should  be 
taken  to  hold  the  specimen  so  that  the  new  marks  will  cross  the  old  ones  at  right 
angles  and  the  grinding  should  be  continued  until  the  emery-wheel  marks  have  been 
completely  erased.  After  washing  the  specimen  it  is  ready  for  treatment  on  the 
next  surface  covered  with  broadcloth  upon  which  has  been  spread  tripoli  powder 
and  water,  here  again  turning  the  sample  90  degrees.  When  the  marks  left  by  the 
preceding  operation  have  been  removed,  the  specimen  is  washed  and  given  the  final 
polishing  treatment  by  pressing  it  lightly  upon  the  other  side  of  the  cast-iron  disk 
upon  which  rouge  and  water  is  used.  The  various  polishing  powders  mixed  with 
water  may  be  conveniently  applied  to  their  respective  disks  by  means  of  flat  and 
rather  stiff  brushes.  The  surface  of  a  properly  polished  sample  should  be  highly 
specular  and  free  from  scratches. 

Time  will  be  saved  by  exerting  a  slight  pressure  only  while  polishing,  especially 
on  the  emery-wheel  and  emery  disk,  because  deep  marks  due  to  these  abrasers  will 
be  troublesome  to  remove  with  the  finer  powders.  With  these  machines  a  sample  of 
steel  measuring  %  in.  square  or  J^  in.  in  diameter  is  readily  polished  in  10  minutes. 

Etching.  —  If  the  polished  sample  of  wrought  iron  were  now  placed  under  the 
microscope,  it  would  be  possible  to  detect  some  of  the  slag  particles  but  the  structure 
of  the  iron  itself  could  not  be  seen,  because  all  parts  of  the  sample  being  uniformly 
bright  would  reflect  the  light  to  the  same  extent.  To  make  the  structure  apparent 
under  the  microscope  it  is  necessary  to  impart  unlike  appearances  to  the  constituents. 
This  is  generally  accomplished  by  producing  a  slight  corrosion  or  etching  of  the  pol- 
ished surface.  For  this  purpose  acid  solutions  are  generally  used  which  attack  some 
constituents  more  deeply  than  others  or  to  the  exclusion  of  others,  which  action  may 
or  may  not  be  accompanied  by  the  deposition  of  some  precipitated  matter. 

Arnold  considers  the  operation  of  etching  with  dilute  acids  to  be  of  an  electro- 
lytic nature,  some  of  the  constituents  being  electro-negative  to  others,  hence  the 
attack  of  some  of  these  (electro-positive  constituents)  to  the  exclusion  of  others 
(electro-negative  constituents)  and  the  darker  coloration  of  the  former. 

The  various  methods  which  have  been  used  or  recommended  for  the  development 
of  the  structure  of  iron  and  steel  samples  will  be  found  duly  described  in  an  appendix 
to  these  lessons  but  only  those  methods  which  in  the  author's  opinion  are  most  satis- 
factory will  be  mentioned  in  connection  with  the  experiments  of  this  book. 

Etching  with  Picric  Acid.  (Igevsky.)  —  An  etching  solution  should  be  prepared 
containing  5  grams  of  picric  acid,  chemically  pure,  and  95  cubic  centimeters  of 
absolute  alcohol.  This  should  be  kept  in  a  well-stopped  glass  bottle. 


LESSON  III  — WROUGHT  IRON  7 

A  small  amount  of  this  solution  should  be  poured  in  a  glass  or  porcelain  dish, 
preferably  a  small  crystallizing  glass  dish  with  cover,  and  the  sample  immersed  in  it 
for  30  seconds,  when  it  should  be  removed,  conveniently  with  a  pair  of  pincers  (pref- 
erably with  platinum  tips),  and  washed  in  alcohol.  The  sample  should  now  be  dried, 
preferably  by  means  of  a  blast  for  which  a  foot-blower  will  answer  very  well.  After 
rubbing  the  sample  very  gently  once  or  twice  upon  a  smooth  piece  of  chamois  leather 
stretched  on  a  wooden  block  and  carefully  kept  free  from  dust,  it  will  be  ready  for 
examination. 

Examination.  —  The  prepared  sample  should  be  suspended  to  the  magnetic 
specimen  holder  described  under  "Apparatus"  in  such  a  way  as  tc^ expose  to  view  as 
much  as  possible  of  its  surface.  The  source  of  light  and  condensers  should  be  ad- 
justed so  that  a  beam  of  light  of  suitable  size  enters  the  vertical  illuminator ;  the 
light  beam  should  cover  a  little  more  than  the  aperture  of  the  illuminator.  A  2  in. 
(5X)  eyepiece  and  a  16  mm.  (%  in.)  objective  will  be  a  satisfactory  combination  for 
the  examination.  The  image  of  the  specimen  should  be  focussed  roughly  by  the  rack 
and  pinion  motion  of  the  stage,  and  the  milled  head  of  the  vertical  illuminator  turned 
tentatively  and  gently  right  and  left  until  the  sample  appears  brightly  and  uniformly 
lighted.  The  object  should  now  be  brought  to  a  sharp  focus  by  means  of  the  fine 
adjustment. 

The  etching  treatment  should  have  outlined  the  joints  between  the  ferrite  grains 
clearly  and  sharply.  If  the  structure  lacks  clearness  it  is  safe  to  infer  that  the  etch- 
ing was  not  properly  done.  In  that  case  the  sample  should  be  rubbed  a  few  times  on 
the  chamois  leather  block  and  again  examined  without  repeating  the  etching.  If 
the  structure  remains  ill-defined,  rub  the  specimen  a  minute  or  two  on  the  rouge 
block  or  disk,  wash,  dry,  and  repeat  the  etching  treatment  until  satisfactory  results 
are  obtained. 

Should  the  boundaries  of  the  ferrite  grains  appear  too  faint,  the  etching  treat- 
ment should  be  repeated  without  repolishing,  so  as  to  etch  these  lines  more  deeply. 

As  the  usual  purpose  of  the  microscopical  examination  of  samples  of  wrought 
iron  is  to  ascertain  the  quantity  and  mode  of  occurrence  of  the  slag  and  the  dimen- 
sions of  the  ferrite  grains,  it  is  not  generally  desired  to  etch  the  sample  so  deeply 
that  some  of  the  grains  become  deeply  colored,  still  less  that  etching  pits  begin  to 
appear. 

In  this  experiment,  however,  the  student  is  advised  to  etch  his  sample  gradually 
so  that  the  different  stages  of  the  structure  may  be  clearly  seen: —  (1)  before  etching: 
slag  fibers  and  a  brilliant  structureless  matrix,  (2)  after  a  slight  etching:  ferrite  grains 
sharply  defined  but  remaining  uncolored  or  but  slightly  colored,  (3)  after  a  deeper 
etching:  some  of  the  ferrite  grains  deeply  colored,  and  (4)  after  a  still  deeper  etching: 
small  cubic  etching  pits  beginning  to  appear. 

The  production  of  these  etching  pits,  however,  is  often  a  troublesome  and  uncer- 
tain operation.  Heyn  recommends  for  that  purpose  etching  with  double  chloride  of 
copper  and  ammonium,  others  (Stead)  "a  sufficiently  long  immersion  in  lukewarm 
20  per  cent  sulphuric  acid,  followed  by  cleaning  in  nitric  acid." 

Etching  with  Diluted  Nitric  Acid.  —  The  sample  used  in  the  above  experiment 
should  be  rubbed  a  short  while  on  the  rouge  block  so  as  to  remove  the  effect  of  the 
etching,  washed  and  dried  in  the  usual  way  and  etched  with  a  solution  containing 
10  c.c.  of  concentrated,  chemically  pure  nitric  acid  and  90  c.c.  of  absolute  alcohol. 
The  etching  should  be  conducted  in  the  same  way  but  as  this  reagent  acts  more 


8  LESSON  III  — WROUGHT  IRON 

quickly  the  sample  should  not  be  left  in  the  solution  more  than  10  or  15  seconds, 
when  it  should  be  washed  in  alcohol,  carefully  dried  and  passed  gently  once  or  twice 
over  the  chamois  leather  block. 

If  the  boundaries  of  the  grains  are  too  faintly  developed  the  etching  should  be 
repeated  without  repolishing. 

The  appearance  of  the  sample  after  this  treatment  should  be  identical  to  that 
resulting  from  etching  with  picric  acid,  the  only  essential  difference  between  these 
two  reagents  being  the  slower  action  of  the  latter. 

Etching  with  Concentrated  Nitric  Acid.  (Sauveur.)  —  The  corrosion  due  to  the 
last  treatment  should  be  removed  by  rubbing  the  sample  on  the  rouge  block  which 
after  careful  washing  and  drying  should  now  be  etched  with  concentrated  nitric  acid 
as  follows:  The  polished  specimen  conveniently  held  with  a  pair  of  pincers  (prefer- 
ably with  platinum  tips)  should  be  dipped  in  a  beaker  or  other  vessel  containing 
concentrated  nitric  acid  (1.42  specific  gravity)  and  immediately  afterwards  held 
under  an  abundant  stream  of  running  water.  When  iron  is  immersed  in  concen- 
trated nitric  it  assumes  the  passive  state,  that  is,  it  is  not  affected  by  the  acid.  As 
soon,  however,  as  the  layer  of  concentrated  acid  which  covers  the  polished  surface 
is  diluted  by  the  running  water,  the  steel  is  vigorously  attacked  but  for  so  short  a 
time  (since  the  water  soon  removes  all  traces  of  acid)  that  there  is  little  danger  of 
etching  too  deeply.  One  such  treatment  is  generally  sufficient  to  bring  out  the  struc- 
ture sharply  and  clearly  but  if  the  specimen  is  found  insufficiently  etched,  the  etch- 
ing should  be  repeated  in  exactly  the  same  manner.  The  author  believes  that  the 
simplicity  of  this  etching  treatment  and  the  excellent  results  generally  obtained 
have  been  overlooked  by  metallographists. 

Transverse  Section  of  Wrought-Iron  Bar.  —  The  student  should  prepare  a  trans- 
verse section  (preferably  not  over  y%  in.  thick)  of  the  same  wrought-iron  bar,  following 
exactly  the  manipulations  described  for  the  polishing  and  etching  of  the  longitudinal 
section.  He  should  compare  the  structure  of  the  two  sections  and  notice  (1)  their 
similarity  as  to  the  appearance  of  the  ferrite  grains  and  (2)  the  unlike  occurrence  of 
the  slag  which  in  the  transverse  section  is  present  as  small  irregular  areas  correspond- 
ing to  cross  sections  of  the  slag  fibers  of  the  longitudinal  section. 


Examination 

I.     Describe  briefly  the  structure  of  commercial  wrought  iron,  explaining  the  dif- 
ference between  the  appearances  of  longitudinal  and  transverse  sections. 

II.     Describe  the  structure  of  your  samples  and  mention  any  difficulty  which  you 
may  have  encountered  in  your  manipulations. 

III.     If  you  have  any  preference  for  one  of  the  etching  methods  described  give  your 
reasons  in  support  of  it. 


LESSON  IV 

LOW  CARBON  STEEL 

In  this  and  the  following  lessons  steel  will  be  considered  as_a^pAire  alloy  of  iron 
and  carbon,  i.e  free  from  the  impurities  (silicon,  manganese,  sulphur,  and  phosphorus) 
always  present  in  commercial  products.  The  influence  of  these  elements  upon  the 
structure  of  steel  will  form  the  subject  of  another  lesson. 

Normal  Structure.  —  The  structures  described  in  this  and  the  next  lesson  refer  to 
the  condition  of  steel  after  forging  followed  by  heating  to,  and  slowly  cooling  from, 
a  high  temperature  (900  to  1000  deg.  C.).  Such  treatments,  for  reasons  that  will 
be  understood  later,  promote  soundness,  remove  internal  strains,  prevent  excessive 
coarseness  of  structure  (as  in  castings),  and  permit  a  state  of  stable  equilibrium  to  be 
assumed  by  the  constituents.  The  resulting  structure  may  be  conveniently  called 
the  "normal"  structure  and  it  will  be  so  called  in  these  lessons. 

Grading  of  Steel  vs.  Carbon  Content.  —  Steel  is  generally  graded  according  to 
the  amount  of  carbon  it  contains.  The  following  terms  are  those  most  commonly 
used : 

Very  low  carbon  steel,  very  mild  or  extra  mild  steel, 

very  soft  or  dead  soft  steel carbon  not  over  0.10  per  cent 

Low  carbon  steel,  mild  steel,  soft  steel carbon  not  over  0.25  per  cent 

Medium  high  carbon  steel,  half  hard  steel     ....  carbon  0.26  to  0.60  per  cent 

High  carbon  steel,  hard  steel       carbon  over  0.60  per  cent 

Very  high  carbon  steel,  very  hard  or  extra  hard  steel  carbon  over  1.25  per  cent 

This  classification  is  somewhat  arbitrary  as  there  are  no  sharp  lines  of  demarcations 
universally  recognized  between  the  various  grades. 

It  will  be  seen  in  another  lesson  that  steel  containing  about  0.85  per  cent  carbon 
is  also  known  as  eutectoid  steel,  steel  containing  less  carbon  as  hypo-eutectoid  steel 
and  more  highly  carburized  metal  as  hyper-eutectoid  steel. 

Low  Carbon  Steel  vs.  Wrought  Iron.  —  As  already  mentioned  the  distinction 
between  low  carbon  steel  and  wrought  iron  is  based  upon  the  difference  between  the 
methods  employed  for  their  respective  manufacture  rather  than  upon  unlike  chemical 
or  physical  properties,  for  these  metals  may  indeed  be  quite  identical  both  physically 
and  chemically.  The  mere  melting  of  wrought  iron  would  undoubtedly,  in  accord- 
ance with  the  universally  accepted  definition  of  steel,  convert  it  into  steel  since  we 
would  now  have  a  malleable  metal  initially  cast.  Such  treatment  would  of  course 
result  in  the  elimination  of  the  slag  mechanically  retained  by  the  wrought  iron:  the 
melted  metal  would  be  slagless,  barring  cemented  steel,  another  essential  property  of 
steel.  Since  wrought  iron  generally  contains  but  a  small  amount  of  carbon,  melting 
it  would  convert  it  into  low  carbon  steel. 

i 


2  LESSON  IV  — LOW  CARBON  STEEL 

The  Structure  of  Low  Carbon  Steel.  —  From  the  above  considerations  regarding 
the  resemblance  between  wrought  iron  and  low  carbon  steel,  the  structure  of  the 
latter  may  fairly  be  anticipated.  Seeing  that  low  carbon  steel  may  be  considered  as 
wrought  iron  from  which  the  mechanically  held  slag  has  been  expelled  through  melt- 


Fig.  1.  —  Steel.    Carbon  0.08  per  cent. 
Magnification  not  stated.    (Arnold.) 


Fig.  2.  —  Steel.    Carbon  about  0.20  per  cent. 
Magnified  200  diameters.     (Guillet.) 


ing,  we  should  expect  the  absence  of  slag  to  be  the  only  marked  difference  between 
the  structure  of  low  carbon  steel  and  that  of  wrought  iron. 

The  microstructure  of  low  carbon  steel  in  the  case  of  samples  containing  respec- 
tively about  0.10  and  0.20  per  cent  carbon  is  illustrated  in  Figures  1  to  5.    It  will  be 


ill  "&^i& 


-M.;y,-  '.,V:-Y 


Fig.  3.  —  Steel.    Carbon  0.10  per  cent.     Magnified  100 
diameters.     (Boynton.) 

seen  (Figs.  1,  2,  and  3)  to  consist  chiefly  of  a  mass  of  ferrite  (carbonless  iron)  exhibiting 
the  usual  polyhedral  crystalline  grains  described  in  preceding  lessons.  The  ferrite  pres- 
ent in  low  carbon  steel  is  similar  in  every  respect  to  the  ferrite  of  wrought  iron.  At 
the  junctions  of  many  ferrite  grains,  however,  some  dark  areas  will  be  noted,  an  evi- 


LESSON  IV  — LOW   CARBON  STEEL  3 

dence  of  the  existence  in  the  metal  of  another  constituent.  Since  ferrite  is  practically 
free  from  carbon,  it  is  evident  that  the  carbon  present  in  the  steel  must  have  segre- 
gated into  these  small  dark  masses.  As  to  the  exact  nature  of  this  dark  constituent 
it  will  be  apparent  that  it  cannot  consist  of  pure  carbon  for  it  is  well  known  that  the 
carbon  present  in  steel  does  not  exist  in  the  free  state  but  on  the  contrary  is  combined 
with  some  of  the  iron  forming  a  definite  chemical  compound  or  carbide  of  iron  whose 
formula  is  FesC.1  This  iron  carbide  must  necessarily  be  located  in  the  dark  areas, 
but  are  these" made  up  exclusively  of  this  carbide?  To  find  an  answer  to  this  question 
let  us  examine  the  structure  of  steel  under  a  higher  magnification  (Figs.  1  and  5). 
This  reveals  the  existence  of  two  components  in  each  dark  particle  occurring  as  small 
wavy  or  curved  parallel  plates  or  lamellae  alternately  dark  and"  white.  As  to  the 
nature  of  these  two  components,  it  is  evident  that  one  of  them  must  be  the  carbide 
FesC  and  the  other  necessarily  iron  or  ferrite,  since  according  to  the  proximate  analy- 
sis of  steel,  these  are  the  only  two  constituents  which,  to  the  best  of  our  knowledge, 
are  present  in  pure  unhardened  carbon  steel. 

Pearlite.  —  Howe  named  the  microscopical  constituent  just  described  pearlite 
(originally  written  pearlyte)  following  in  this  Dr.  Sorby  who  was  the  first  observer  to 
describe  it  and  who  had  proposed  for  it  the  name  of  "pearly  constituent"  because  it 
frequently  exhibits  a  display  of  colors  very  suggestive  of  mother-of-pearl,  especially 
when  viewed  by  oblique  illumination.  This  appearance  is  due  to  the  fact  that  these 
plates  are  extremely  thin,  seldom  measuring  over  25000  of  an  inch  in  thickness,  and 
that  the  plates  of  carbide,  being  much  harder  than  the  ferrite  plates,  stand  in  relief 
after  polishing,  resulting  in  an  arrangement  very  similar  to  the  refraction  gratings  of 
physicists.  Mother-of-pearl  likewise  is  made  up  of  very  thin  alternate  plates  of  dif- 
ferent colors  and  possibly  of  different  hardness.  The  carbide  plates  remain  bright 
not  being  affected  by  the  usual  etching  reagents,  while  the  ferrite  plates  appear  dark 
because  of  their  being  somewhat  tarnished  by  the  etching  and  also  because,  being 
depressed  owing  to  their  greater  softness,  they  stand  in  the  shadow  of  the  carbide 
plates.  It  will  be  shown  in  another  lesson  that  in  many  series  of  alloys  of  two  metals 
the  alloy  of  lowest  melting-point  called  the  "eutectic"  alloy,  nearly  always  exhibits 
a  composite  structure  like  that  of  pearlite,  i.e.  made  up  of  parallel  plates  alternately 
of  one  and  the  other  constituents.  It  will  also  be  shown  that  in  spite  of  this  very 
great  structural  resemblance  pearlite  is  not  a  true  eutectic  alloy.  Howe  proposed 
to  call  "eutectoid"  the  kind  of  mechanical  mixture  found  in  pearlite  and  this  most 
appropriate  term  has  been  universally  adopted. 

Because  of  the  minute  dimensions  of  the  lamellae  of  pearlite  a  high  magnifica- 
tion, generally  not  less  than  250  or  300  diameters,  is  required  for  its  resolution. 

It  should  be  stated  here  that  pearlite  does  not  always  assume  such  a  distinctly 
laminated  structure.  In  many  instances  its  structure  remains  ill  defined  or  has  a 
granular  rather  than  a  lamellar  appearance,  while  its  behavior  towards  the  etching 
reagents  likewise  varies.  It  will  be  shown  at  the  proper  time  that  this  is  due  to  the 
treatments  to  which  steel  may  be  subjected  and  that  the  exact  nature  of  these  ill-de- 
fined forms  of  pearlite  (often  called  transition  constituents)  has  given  rise  to  a  large 
amount  of  discussion  and  has  been  the  object  of  many  investigations.  It  may  be 

1  The  existence  of  the  carbide  Fe3C  in  unhardened  steel  was  first  shown  in  1885  by  Abel  and 
Muller,  working  independently,  and  has  since  been  confirmed  by  many  other  investigators.  Its 
existence  is  proved  by  dissolving  unhardened  steel  in  a  suitable  solvent  and  analyzing  the  carbona- 
ceous residue. 


4  LESSON  IV  — LOW  CARBON  STEEL 

assumed  for  the  present  that  any  pearlite  which  is  not  distinctly  lamellar  is  not  true 
pearlite. 

It  will  be  noted  that  in  Figure  2  the  pearlite  occupies  about  twice  the  area  covered 
by  the  same  constituent  in  Figure  1.  We  infer  from  this  that  the  amount  of  pearlite 
in  low  carbon  steel  at  least  increases  progressively  with  the  carbon  content.  Doub- 
ling the  amount  of  carbon  doubles  of  course  the  proportion  of  the  iron  carbide  in  the 
steel,  and  since  the  amount  of  pearlite  is  apparently  also  doubled  it  follows  that  iron 
carbide  and  ferrite  must  unite  with  each  other  in  fixed  ratio  to  form  pearlite,  in  other 
words  that  pearlite  always  contains  the  same  proportion  of  carbide  and  hence  also 
of  carbon.  The  accuracy  of  this  conclusion  will  soon  be  shown. 

Free  Ferrite.  —  To  distinguish  between  the  ferrite  included  in  pearlite  and  the 
ferrite  forming  the  balance  of  low  carbon  steel,  the  latter  is  sometimes  called  "free" 


Fig.  4.  —  Steel.    Structure  of  pearlite.     Magnified 
1000  diameters.    (Osmond.) 


Fig.  5.  —  Steel.  Hypo-eutectoid. 
Magnified  750  diameters.  Pear- 
lite  particles  and  surrounding 
ferrite.  (Goerens.) 


ferrite,  "structurally  free"  ferrite,  "excess"  ferrite,  "massive"  ferrite,  "non-eutec- 
toid"  or  "pro-eutectoid"  ferrite,  "surplus"  ferrite.  In  these  lessons  it  will  be  referred 
to  as  free  ferrite  while  the  ferrite  forming  part  of  the  pearlite  will  be  called  pearlite- 
ferrite.  Some  writers  refer  to  the  latter  as  eutectoid-ferrite. 

In  the  absence  of  any  conclusive  evidence  to  the  contrary,  it  is  natural  to  infer 
that  free  ferrite  and  pearlite-ferrite  are  identical,  that  is,  pure  iron  in  pure  steel  and 
iron  holding  in  solution  small  quantities  of  silicon  and  phosphorus,  and  possibly  of 
other  impurities,  in  impure  (commercial)  steel. 

This  has  been  doubted  by  some  writers,  however,  who  have  noted  that  the  ferrite  of  some  pear- 
lites  was  more  readily  colored  on  etching  than  free  ferrite  and  they  saw  in  this  and  in  some  other 
evidences  an  indication  that  pearlite-ferrite  may  be  less  pure  than  free  ferrite.  Benedicks,  for  in- 
stance, believes,  or  at  least  believed  at  one  time,  that  the  pearlite-ferrite  of  some  steels  could  con- 
tain as  much  as  0.27  per  cent  of  carbon  dissolved  in  beta  iron,  whereas  free  ferrite  is  in  the  alpha 
condition.  This  carburized  and  allotropic  ferrite  Benedicks  called  "ferronite." 


LESSON   IV  — LOW  CARBON   STEEL  5 

Cementite.  —  The  name  of  cementite  has  been  given  by  Howe  to  the  carbide 
Fe3C  and  universally  adopted.  The  term  is  derived  from  "cement"  steel  (cementa- 
tion steel,  blister  steel,  converted  steel)  which  being  generally  a  high  carbon  steel 
contains  a  great  deal  of  this  carbide,  that  is,  of  cementite. 

According  to  the  atomic  weights  of  iron  (56)  and  of  carbon  (12)  cementite  must 
contain 

12  x  100 

-  =  6.67  per  cent  carbon 
3  x  56  +  12 

The  carbon  present  in  cementite  is  frequently  referred  to  as  "cement"  carbon, 
occasionally  as  carbide  carbon,  to  distinguish  it  from  other  forms^pf  carbon  found  in 
iron  and  steel  and  to  be  described  later  (hardening  carbon,  graphitic  carbon,  temper 
carbon,  etc.). 

Cementite  is  an  extremely  hard  substance,  being  in  fact  the  hardest  of  all  the 
constituents  occurring  in  iron  and  steel,  harder  even  than  hardened,  high  carbon  steel. 
Howe  states  that  it  is  harder  than  glass  and  nearly  as  brittle.  As  it  scratches  feldspar 
but  not  quartz  it  is  generally  assigned  to  rank  6  or  6.5  in  the  Mohs  scale  of  hardness.1 

It  will  be  shown  later  that  when  steel  contains  an  appreciable  amount  of  manga- 
nese, as  is  nearly  always  the  case  in  commercial  products,  a  portion  at  least  of  this 
manganese  also  forms  anapaa  carbide  Mn3C  and  that  this  carbide  unites  with  the  iron 
carbide  Fe3C  to  form  cementite.  It  is  well  to  bear  in  mind,  therefore,  that  in  com- 
mercial steel  cementite  generally  contains  besides  Fe3C  varying  amounts  of  this  car- 
bide of  manganese.  As  the  atomic  weight  of  manganese  is  nearly  the  same  as  that  of 
iron,  55  compared  to  56,  it  so  happens  that  the  presence  of  manganese  in  cementite 
affects  but  very  little  its  carbon  content,  which  for  all  practical  purposes  may  be 
taken  as  6.67  regardless  of  the  amount  of  manganese  it  may  contain.  Cementite 
containing  much  manganese  has  been  called  manganiferous  cementite  by  some  writers. 

Whether  any  portion  of  the  other  impurities  present  in  iron  and  steel  (sulphur, 
silicon,  phosphorus)  is  ever  included  in  cementite  is  not  positively  known  but  in  the 
absence  of  indications  to  the  contrary  it  is  generally  assumed  that  cementite  is  free, 
practically  at  least,  from  these  metalloids. 

Cementite  generally  remains  bright  and  brilliant  after  the  ordinary  etching  treat- 
ments employed  to  reveal  the  structure  of  steel.  It  will  be  shown  later,  however, 
that  some  special  reagents  may  be  used  which  color  it  deeply. 


Experiments 

The  student  should  procure  samples  of  forged  steel  containing  respectively  about 
0.10  and  0.20  per  cent  carbon.  These  should  be  heated  to  1000  deg.  C.  and  slowly 
cooled,  preferably  with  the  furnace  in  which  they  were  heated. 

Polishing.  —  Specimens  should  be  cut  from  these  samples  of  suitable  size  for 
microscopical  examinations  (preferably  not  over  }/£  in.  square  or  round  and  J/£  in. 
thick).  These  specimens  should  be  polished  for  examination  in  accordance  with  the 
instruction  given  in  Lesson  III,  taking  care  to  prepare  a  freshly  cut  surface,  that  is  a 
portion  of  the  sample  which  did  not  suffer  from  decarburization  in  the  furnace. 

1  The  Mohs  scale  is  as  follows,  beginning  with  the  softest  and  ending  with  the  hardest  mineral 
and  each  mineral  being  capable  of  scratching  the  preceding  ones:  (1)  Talc,  (2)  Gypsum,  (3)  Calcite, 
(4)  Fluorite,  (5)  Apatite,  (6)  Feldspar,  (7)  Quartz,  (8)  Topaz,  (9)  Corundum,  and  (10)  Diamond. 


6  LESSON  IV  — LOW  CARBON  STEEL 

Etching.  —  These  samples  should  be  etched  successively  with  (1)  a  solution  of 
picric  acid  in  absolute  alcohol,  (2)  a  solution  of  nitric  acid  in  absolute  alcohol,  and 
(3)  concentrated  nitric  acid,  following  the  instructions  for  etching,  washing,  drying, 
etc.,  given  in  Lesson  III.  They  should  be  carefully  examined  after  each  etching  and 
the  treatment  repeated  in  case  the  structure  does  not  appear  sharply  and  clearly 
defined. 

Examination.  —  The  prepared  specimens,  suspended  to  the  magnetic  holder, 
should  first  be  examined  with  a  low  power  objective  (^  in.  or  16  mm.)  and  eyepiece 
(2  in.  or  5X)  a  combination  which  will  yield  a  magnification  of  about  50  diameters. 
The  central  portion  of  the  specimens  should  be  observed  and  their  structure  com- 
pared with  the  illustrations  of  similar  steels  reproduced  in  this  lesson.  It  will  be  in- 
structive to  examine  also  the  edges  of  the  samples  and  to  note  that  the  outside  of 
the  bars  have  been  somewhat  decarburized  through  the  heating  operation,  unless 
indeed  the  bars  had  been  effectively  protected  against  oxidation.  This  decarburiza- 
tion  will  be  apparent  from  a  decrease  in  the  proportion  of  pearlite. 

The  contrast  between  the  bright  ferrite  and  the  dark  areas  of  pearlite  should  be 
very  marked,  and  the  junction  lines  between  the  ferrite  grains  should  appear  like  a 
delicate  but  distinct  network.  If  these  appearances  lack  intensity  the  etching  treat- 
ment should  be  repeated  without  repolishing.  While  a  deeper  etching,  however,  will 
bring  out  more  distinctly  the  junction  lines  between  the  ferrite  grains,  it  will  some- 
what blur  the  structure  of  pearlite.  If  the  structure  is  ill  defined  the  sample  should 
be  rubbed  a  short  while  on  the  rouge  block  or  disk  and  the  etching  repeated.  The 
most  satisfactory  etching  is  the  one  which  will  show  great  contrast  between  the  ferrite 
and  pearlite,  while  bringing  out  somewhat  faintly  the  ferrite  grains. 

To  reveal  the  composite  structure  of  pearlite  a  higher  magnification  is  needed. 
To  that  end  a  4  mm.  or  }/§  in.  objective  and  a  1  in.  eyepiece  will  be  found  satisfac- 
tory, as  this  will  yield  a  magnification  of  about  430  diameters. 

Examination  under  high  power  requires  careful  adjustment  of  the  light  and  ver- 
tical illufninator  and  careful  focusing.  The  parallel  plates  of  pearlite  should  be 
clearly  seen,  although  it  is  not  always  possible  to  resolve  satisfactorily  every  particle 
of  that  constituent. 

Photomicrography.  —  The  student  should  proceed  to  take  low  power  photomicro- 
graphs of  the  two  samples  of  wrought  iron  and  two  samples  of  steel  which  he  has 
so  far  prepared  and  examined. 

For  the  taking  of  photomicrographs  the  appliances  described  and  illustrated  under 
"Apparatus"  are  recommended. 

After  having  selected  the  spot  to  be  photographed,  light  tight  connection  should 
be  established  between  the  microscope  and  the  camera  (in  the  case  of  the  Metalloscope 
the  camera  and  microscope  are  permanently  connected)  and  the  light  carefully  ad- 
justed so  as  to  obtain  on  the  screen  of  the  camera  as  bright  and  even  an  illumination 
as  possible.  The  image  should  now  be  focused  as  sharply  as  possible,  using  a  focus- 
ing cloth  if  necessary  and  gently  turning  the  fine  adjustment  screw  of  the  stand.  A 
focusing  glass  may  be  used  with  great  advantage  for  this  operation  and  is  of  special 
importance  when  photographing  with  high  power  objectives.  It  should  be  placed  on 
the  plain  glass  circle  which  occupies  the  center  of  the  screen  of  the  camera,  and  the 
image  focused  while  being  viewed  through  this  lens.  By  this  means  we  magnify  the 
image  formed  upon  the  camera  screen,  and  are  therefore  able  to  focuss  it  more  sharply 
in  its  finer  details.  Considerable  light,  however,  is  lost  and  the  object  will  often  ap- 


LESSON  IV  — LOW  CARBON  STEEL  7 

pear  but  dimly  lighted.  The  rule  is  to  secure  the  clearest  possible  image  while  work- 
ing tentatively  the  fine  adjustment  in  both  directions,  bearing  in  mind  that,  at  its 
best,  the  image  may.  appear  blurred  and  dimly  lighted. 

An  ordinary  eyepiece  may  be  used  in  place  of  a  focusing  glass  with,  in  many 
cases,  satisfactory  results. 

Exposure. — After  the  image  has  been  properly  illuminated  and  focused  the  sen- 
sitive plate  should  be  introduced  and  exposed,  with  the  ordinary  precautions,  for  a 
suitable  length  of  time.  The  required  time  of  exposure  will  vary  according  to  (a) 
the  kind  of  photographic  plate  used  and  (6)  the  amount  and  nature  of  light  reaching 
the  plate,  which  in  turn  will  depend  upon  (1)  the  nature  of  the  prepared  surface, 
especially  its  power  to  reflect  light,  (2)  the  kind  of  illumination-used,  (3)  the  position 
of  the  diaphragm  or  diaphragms  controlling  the  amount  of  light  allowed  to  reach  the 
plate,  (4)  the  kind  of  light  filters  used,  if  any,  (5)  the  resolving  and  magnifying  powers 
of  the  combination  of  objective  and  eyepiece  used,  and  (6)  the  distance  between  the 
screen  of  the  camera  and  the  object. 

Specimens  which  after  etching  remain  quite  bright  naturally  reflect  more  light 
and  consequently  require  for  their  photography  a  shorter  time  than  duller  specimens. 
Generally  speaking  the  higher  the  magnification  the  less  light,  hence  the  longer  the 
exposure.  The  use  of  colored  screens  or  solutions  (light  filters)  as  a  rule  lengthens  the 
exposure  considerably.  By  placing  the  screen  of  the  camera  at  a  greater  distance 
from  the  object  (i.e.  by  extending  the  bellows  of  the  camera)  the  magnification  is 
increased  but  with  accompanying  loss  of  light,  and,  therefore,  increased  length  of 
exposure. 

Using  a  rather  slow  plate,  no  screens,  a  combination  of  objective  and  eyepiece 
yielding  a  magnification  of  100  diameters  at  a  distance  of  24  to  30  inches  from  the 
object,  the  diaphragm  being  wide  open,  the  exposure  for  most  iron  and  steel  samples 
would  vary  between  a  fraction  of  a  second  with  a  powerful  arc  lamp  and  some  10  to 
20  minutes  with  a  welsbach  lamp.  The  use  of  higher  magnifications,  of  screens,  and 
the  closing  of  the  diaphragm  may  lengthen  the  exposure  to  such  an  extent  as  to  re- 
quire one  minute  or  more  with  an  arc  lamp  and  one  hour  or  more  with  a  welsbach 
lamp. 

The  use  of  sources  of  light  of  greater  intensity  than  the  welsbach  mantle  but 
less  intense  than  the  electric  arc,  such  as  the  Nernst  lamp,  the  acetylene  lamp,  or 
the  oxy-hydrogen  lamp,  calls  for  exposures  of  intermediate  lengths  between  the  two 
extremes  considered  in  the  preceding  paragraph. 

Diaphragms  and  Shutters.  —  It  is  sometimes  advantageous  to  be  able  to  control 
the  pencil  of  light  entering  the  illuminator  with  a  view  of  securing  sharper  definition. 
To  that  effect  an  iris  diaphragm  suitably  mounted  should  be  placed  between  the 
condensing  lens  or  lenses  and  the  vertical  illuminator.  Some  sort  of  an  automatic 
shutter  is  convenient  to  control  the  exposure  of  the  plates.  This  shutter  may  ad- 
vantageously be  combined  with  the  iris  diaphragm.  Instead  of  being  placed  between 
the  source  of  light  and  the  vertical  illuminator,  diaphragms  and  shutters  are  some- 
times inserted  between  the  camera  and  the  microscope.  The  best  disposition  con- 
sists in  placing  an  iris  diaphragm  between  the  condensing  lenses  and  the  vertical 
illuminator,  thus  controlling  the  amount  of  light  entering  the  latter,  and  another, 
diaphragm  combined  with  automatic  shutter  between  the  camera  and  microscope  tube. 

Monochromatic  Light.  —  The  different  sources  of  light  used  for  microscopical 
work  yield  white  light  and  since  the  correction,  even  of  apoehromatic  objectives,  for 


LESSON   IV  — LOW  CARBON   STEEL 

chromatic  aberration  is  never  perfect,  it  is  evident  that  the  use  of  monochromatic 
light,  i.e.  light  of  one  wave  length,  is  theoretically  preferable,  especially  for  photo- 
graphing. 

Monochromatic  light  may  be  obtained  in  two  ways :  (a)  by  using  a  source  of  light 
actually  monochromatic  and  (6)  by  causing  white  light  to  pass  through  colored  glass 
screens  or  colored  solutions  (light  filters),  preventing  the  passage  of  some  undesirable 
rays.  The  mercury  arc  lamp  yields  a  nearly  monochromatic  light  and  has  been  tried 
by  Le  Chatelier  with  satisfactory  results.  It  seems  more  convenient,  however,  when 
monochromatic  light  is  wanted,  to  use  light  filters  of  suitable  colors,  in  which  case 
colored  glass  screens  will  be  found  easier  to  manipulate  than  glass  cells  containing 
colored  solutions. 

The  beginner  is  advised  to  dispense  with  the  use  of  colored  screens  or  other  ray 
filters  until  he  has  acquired  experience  and  skill  in  taking  photomicrographs,  when  he 
will  be  better  qualified  to  judge  of  their  merits  and  to  employ  them  intelligently. 

Photographic  Plates.  —  The  use  of  so-called  "Process"  or  "Contrast"  plates  is 
recommended.  These  plates  are  slow  but  generally  yield  negatives  with  sharp  con- 
trasts. Orthochromatic  plates  may  also  be  used  with,  in  some  instances,  excellent 
results.  These  plates  are  much  more  rapid  but  as  they  call  for  the  use  of  a  colored 
screen,  the  time  of  exposure  may  be  even  longer  than  with  the  slower  kind. 

Development.  —  Formula  and  directions  accompany  each  box  of  plates  and  the 
student  could  not  do  better  than  to  follow  them  faithfully. 

Printing.  —  Any  printing  out  or  developing  paper  may  be  used,  the  printing,  de- 
veloping, or  toning  being  conducted  in  the  usual  way.  Drying  on  ferrotype  plates 
affords  a  quick  means  of  finishing  the  prints  and  giving  them  a  satisfactory  luster. 
It  is  recommended  to  trim  the  prints  round,  2  to  2^  inches  in  diameter,  by  means 
of  a  margin  trimmer  and  suitable  circular  forms,  as  this  will  give  them  a  very  neat 
appearance. 

Mounting.  —  It  is  well  to  paste  the  prints  on  suitable  cardboard  mounts  afford- 
ing room  for  the  recording  of  useful  data.  A  very  satisfactory  mount  has  been 
illustrated  under  "Apparatus." 

Examination 

I.     Describe  the  structure  of  low  carbon  steel  and  more  especially  of  pearlite. 
II.     What  is  free  ferrite? 

III.     Describe  the  structure  of  your  samples  and  mention  any  difficulty  encountered 
in  polishing,  etching,  or  photographing  them. 


LESSON  V 

MEDIUM   HIGH  AND   HIGH   CARBON   STEEL 

Medium  High  Carbon  Steel.  —  The  normal  structure  of  steel  (i.e.  its  structure 
after  forging,  reheating  to  a  high  temperature  and  slow  cooling)  containing  about 
0.30  per  cent  carbon  is  illustrated  by  a  drawing  in  Figure  1  and  by  a  photomicro- 
graph in  Figure  2.  It  will  be  noted,  on  comparing  this  structure  to  that  of  lower 
carbon  steels  (Lesson  IV),  that  the  introduction  of  more  carbon  in  the  iron  has  re- 
sulted, as  would  be  expected,  in  the  occurrence  of  a  greater  amount  of  pearlite  and  of 
a  correspondingly  smaller  proportion  of  ferrite.  The  pearlite  occupies  now  roughly 
about  one  third  of  the  total  area.  The  junction  lines  between  the  grains  of  ferrite 


Fig.  1.  —  Steel.    Carbon  0.38  per  cent. 

Magnification  not  stated. 

(Arnold.) 


Fig.  2.  —  Steel.  Carbon  0.33  per  cent.  Mag- 
nified 100  diameters.  Heated  to  1000  deg. 
C.  and  slowly  cooled  in  furnace.  (Hall.) 


should  be  noted.     Under  sufficiently  high  power  the  pearlite  areas  exhibit  the  char- 
acteristic lamellar  structure  described  in  Lesson  IV. 

On  further  addition  of  carbon,  the  amount  of  pearlite,  which  is  evidently  propor- 
tional to  the  percentage  of  carbon,  increases  correspondingly,  as  shown  in  Figures  3 
and  4  illustrating  the  microstructure  of  steel  containing  about  0.50  per  cent  carbon. 
The  pearlite  occupies  here  over  one  half  of  the  total  area.  It  will  be  noticed  that 
the  ferrite  areas  are  only  occasiqnally  resolved  into  polyhedral  grains,  apparently 
because  the  ferrite  now  occurs  in  particles  often  too  small  to  be  made  up  of  several 
crystalline  grains.  These  small  masses  of  ferrite,  however,  are  still  made  up  of  crys- 
talline matter  as  described  and  illustrated  in  Lesson  II.  A  high  power  photomicro- 

1 


LESSON  V  — MEDIUM   HIGH  AND  HIGH  CARBON  STEEL 


graph  of  0.45  per  cent  carbon  steel  is  shown  in  Figure  5.     The  laminations  of  pearlite 
are  clearly  seen. 


Fig.  3.  —  Steel.    Carbon  0.59  per  cent. 

Magnification  not  stated. 

(Arnold.) 


Fig.  4.  —  Steel.  Carbon  0.50  per  cent.  Mag- 
nified 100  diameters.  Heated  to  1000  dcg. 
C.  and  slowly  cooled  in  furnace.  (Burger.) 


Fig.  5.  — •  Steel.    Carbon  0.45  per  cent.    Magnified 
1000  diameters.      (Osmond.) 

When  steel  contains  but  a  small,  although  appreciable,  amount  of  ferrite,  as  is 
the  case  with  carbon  contents  between  0.50  and  0.70  per  cent  the  ferrite  frequently 
forms  envelopes  or  membranes  surrounding  the  pearlite  grains,  an  arrangement 


LESSON  V  — MEDIUM   HIGH  AND  HIGH  CARBON  STEEL  3 

generally  described  as  a  network  structure  the  pearlite  forming  the  meshes  and  the 
free  ferrite  the  net  proper.  These  pearlite  meshes  are  also  described  as  "cells"  or 
"kernels"  and  the  ferrite  membranes  as  "cell  walls"  or  "shells." 


Fig.  6.  —  Steel.  Carbon  0.50  per  cent.  Magnified 
100  diameters.  Heated  to  1000  deg.  C.  and 
cooled  in  air.  (Burger.) 


Fig.  7.  —  Steel.    Hypo-euteetoid.    (Sorby.) 

It  will  be  shown  later  that  this  network  structure  is  promoted  by  rather  rapid 
cooling  from  a  high  temperature,  as  for  instance  by  cooling  small  pieces  in  air. 

Structures  of  this  type  are  illustrated  in  Figures  6  and  7.  The  latter  illustration 
is  of  special  interest  being  a  reproduction  of  one  of  Sorby's  original  drawings  and 
therefore,  the  first  drawing  of  pearlite  ever  published. 


LESSON  V  — MEDIUM   HIGH  AND  HIGH  CARBON  STEEL 


High  Carbon  Steel.  —  Since  the  introduction  of  increasing  amounts  of  carbon  in 
steel  results  in  the  formation  of  a  correspondingly^ihcreasing  proportion  of  pearlite 
and  decreasing  proportion  of  ferrite,  a  degree  of  carburization  must  necessarily  be 
reached,  when  the  whole  mass  will  be  mlde  up  of  pearlite,  the  ferrite  having  finally 
disappeared.  This  critical  point  in  the  structure  of  steel  is  attained  when  the  metal 
contains  somewhere  between  0.80  and  0.90  per  cent  of  carbon,  exceptionally  pure 
steel  requiring  the  larger  proportion  of  carbon  and  impure  steel  the  smaller  for  the 
complete  disappearance  of  ferrite. 

Eutectoid  Steel.  —  Steel  made  up  exclusively  of  pearlite  is  now  quite  universally 
called  "  ejotectoid "  steel,  after  Howe,  the  name  suggesting  the  great  resemblance 
between  pearTTEe  and  euctectic  alloys,  while,  at  the  same  time,  clearly  indicating 
that  pearlite  is  not  a  real  eutectic  alloy.  Previous  to  Howe's  happy  suggestion 


Fig.  8.  —  Steel.    Carbon  0.89  per  cent. 
Magnification  not  stated.    (Arnold.) 


Fig.  9.  —  Steel.    Eutectoid.    Magnified 
750  diameters.     (Goerens.) 


this  steel  was  commonly  described  as  "eutectic"  or  "saturated"  steel.  It  has  also 
been  termed  "aeolic"  or  "benmutic"  steel  but  these  names  have  now  been  aban- 
doned. The  structure  of  eutectoid  steel  is  illustrated  in  Figures  8  and  9. 

Steel  containing  less  than  0.85  per  cent  carbon  or  thereabout,  and  in  which, 
therefore,  some  free  ferrite  is  present,  is  called  "hypo-eutectoid,"  while  steel  more 
highly  carburized  than  eutectoid  steel  is  called  "hyper-eutectoid."  It  will  be  shown 
presently  that  hyper-eutectoid  steel  contains  free  cementite. 

Hyper-Eutectoid  Steel.  —  The  normal  structure  of  steel  containing  from  1.10  to 
1.50  per  cent  carbon  is  illustrated  in  Figures  10  to  13  both  under  low  and  high  mag- 
nification. These  steels  will  be  seen  to  consist,  like  hypo-eutectoid  steel,  of  two  con- 
stituents, one  of  which  being  pearlite  as  clearly  shown  when  examined  under  high 
power.  The  other  constituent  remain's  bright  after  etching  and  might  at  first  be 
taken  for  ferrite.  Upon  reflection,  however,  it  will  be  evident  that  such  cannot  be  its 
nature.  The  light  constituent  of  hyper-eutectoid  steel  consists  of  cementite  which 
is  now  present  in  excess  over  the  amount  required  to  form  pearlite,  just  as  in  hypo- 


LESSON   V  — MEDIUM   HIGH   AND   HIGH   CARBON   STEEL  5 

eutectoid  steel,  ferrite  is  the  excess  constituent.  It  will  be  evident  that  the  ferrite 
and  cementite  which  constitute  all  grades  of  carbon  steels  combine  with  each  other 
in  suitable  proportions  to  form  pearlite,  leaving,  as  the  case  may  be,  an  excess  of 


Fig.  10.  —  Stool.    Carbon  1.20  per  cent. 
Magnification  not  stated.    (Arnold.) 


Fig.  11.  —  Steel.    Carbon  1.10  per  cent.    Magnified  100  diameters. 
(Boynton.) 

ferrite  (in  hypo-eutectoid  steel)  or  of  cementite  (in  hyper-eutectoid  steel).  A  more 
scientific  explanation  of  the  formation  of  the  normal  structure  of  steel  will  be  offered 
in  a  subsequent  lesson. 

Free  Cementite.  —  To  distinguish  between  the  cementite  forming  part  of  the 
pearlite  (the  bright  plates  of  that  constituent)  and  the  cementite  constituting  the 


6 


LESSON  V  — MEDIUM  HIGH  AND  HIGH  CARBON  STEEL 


balance  of  hyper-eutectoid  steel,  the  latter  is  generally  called  "free"  cementite, 
"structurally  free"  cementite,  "excess"  cementite,  "massive"  cementite,  "non- 
eutectoid"  cementite,  "surplus"  cementite,  while  the  cementite  included  in  the 
pearlite  is  sometimes  referred  to  as  pearlite-cementite  or  eutectoid  cementite.  In 
these  lessons  the  cementite  in  excess  over  the  eutectoid  ratio  will  be  called  free  cemen- 
tite. 

In  the  absence  of  any  conclusive  evidences  to  the  contrary,  and  in  conformity 
with  the  nature  of  eutectic  alloys  in  general,  it  is  assumed  that  free  cementite  and 
pearlite-cementite  are  identical  in  composition  and  properties. 

As  already  stated  in  Lesson  IV  the  cementite  of  commercial  steel  is  not  pure 
Fe3C  but  contains  small  and  varying  amounts  of  Mn3C. 


Fig.  12.  —  Steel.    Carbon  1.43  per  cent. 

(Boynton.) 


Magnified  50  diameters. 


As  shown  in  Figures  10  and  11  hyper-eutectoid  steel  like  hypo-eutectoid  steel 
may  assume  a  network  structure.  In  both  cases  the  meshes  consist  of  pearlite  but 
the  net  proper  which  in  hypo-eutectoid  steel  represents  membranes  of  free  ferrite 
indicate  now  the  occurrence  of  membranes  of  free  cementite. 

Hypo-  vs.  Hyper-Eutectoid  Steel.  —  While  there  is  considerable  similarity  be- 
tween the  structure  of  steel  containing  but  a  slight  excess  of  ferrite  and  the  structure 
of  steel  containing  but  a  slight  excess  of  cementite,  a  little  experience  and  careful 
examination  will  reveal  differences  in  their  appearances  and  properties  which  will 
make  it  possible,  generally,  to  distinguish  between  them.  Cementite  has  a  more 
metallic  luster  than  ferrite  and  remains  bright  and  structureless,  even  after  prolonged 
etching  with  the  ordinary  reagents,  while  ferrite  is  colored  and,  if  present  in  suf- 
ficiently large  masses,  resolved  into  grains  by  such  treatment.  Cementite  is  extremely 
hard,  standing  in  relief,  while  ferrite  being  soft  is  depressed  by  the  polishing  opera- 
tion. Ferrite  is  readily  scratched  by  a  needle  drawn  across  the  polished  surface  while 


LESSON   V  — MEDIUM   HIGH   AND   HIGH   CARBON   STEEL  7 

cementite  remains  unmarked.  Again  it  will  be  noted  that  in  Figures  10  and  12  some 
of  the  pearlite  grains  are  cut  by  plates  or  needles  of  cementite,  independent  of  the  net- 
work of  cementite,  while  when  the  network  consists  of  ferrite,  needles  of  ferrite  are 
frequently  observed  penetrating  the  pearlite  grains,  but  for  the  most  part  connected 
with  the  network  itself  (Fig.  6). 

When  cementite  is  in  excess  the  pearlite  grains  are  generally  smaller  and  the  net- 
work finer  than  is  the  case  with  excess  of  ferrite.  Free  cementite  does  not,  of  course, 
always  assume  the  shape  of  a  fine  network.  It  will  be  shown  in  subsequent  lessons 
that  its  mode  of  occurrence  depends  upon  the  treatment  to  which  the  steel  was  sub- 
jected. 

Etching  of  Cementite.  —  It  has  been  seen  that  cementite  is  not  acted  upon  by  the 
usual  reagents  employed  in  the  etching  of  steel  sections  (picric  acid,  nitric  acid,  tine- 


Fig.  13.  —  Steel.    Carbon  1.43  per  cent. 
(Boynton.) 


Magnified  500  diameters. 


ture  of  iodine,  etc.)  but  that  on  the  contrary  it  remains  brilliant  and  structureless. 
Kourbatoff,  however,  discovered  a  reagent  which  deeply  colors  cementite  while  leav- 
ing the  ferrite  unaffected  (Fig.  14),  thus  affording  a  sure  means  of  distinguishing 
between  the  two.  The  treatment  consists  in  immersing  the  polished  sample  in  a 
boiling  solution  of  sodium  picrate  in  an  excess  of  sodium  hydroxide  for  some  5  to  10 
minutes,  when  the  cementite  assumes  a  brown  to  blackish  coloration.  The  etching 
solution  may  be  prepared  by  adding  2  parts  of  picric  acid  to  98  parts  of  a  solution 
containing  25  per  cent  of  caustic  soda,  for  instance  2  grams  of  picric  acid  in  98  cubic 
centimeters  of  a  solution  made  up  of  24.5  grams  of  caustic  soda  and  73.5  cubic  centi- 
meters of  water. 

More  recently  Matweieff  has  recommended  the  use  of  a  2  per  cent  solution  of 
oxalate  of  ammonium,  used  cold  for  30  minutes,  which  colors  the  cementite  red 
(Fig.  15). 


8 


LESSON   V  — MEDIUM   HIGH   AND   HIGH   CARBON   STEEL 


Carbon  Content  of  Pearlite.  —  The  percentage  of  carbon  in  pearlite,  and,  there- 
fore, in  eutectoid  steel,  has  been  stated  to  be  somewhere  between  0.80  and  0.90  in 
commercial  steel  of  ordinary  quality,  because  when  steel  of  that  degree  of  carburiza- 
tion  is  examined  under  the  microscope  it  is  found  to  be  free  from  any  appreciable 
amount  of  free  ferrite  or  of  free  cementite.  As  the  presence  of  a  very  small  amount 
of  any  of  these  two  constituents  in  the  free  state,  however,  is  very  difficult  to  ascer- 
tain, it  will  be  evident  that  it  is  quite  impossible  to  speak  positively  as  to  the  exact 
amount  of  carbon  needed  to  exclude  both  free  ferrite  and  free  cementite  from  the 
structure.  Moreover  this  carbon  content  of  pearlite  varies  somewhat  with  the  com- 


Fig.  14.  —  Steel.  Hyper-eutectoid.  Free 
cementite  colored  dark  by  sodium  picrate. 
Magnified  500  diameters.  (Guillet.) 


Fig.  15.  —  Steel.  Hyper-eutectoid.  Free  cemen- 
tite colored  dark  by  ammonium  oxalate.  Mag- 
nified 142  diameters.  (Matweieff.) 


position  of  the  steel  and  with  the  treatment  it  has  received.  In  steel  of  ordinary 
commercial  purity  the  eutectoid  point  appears  to  be  in  the  vicinity  of  0.85  per  cent 
carbon. 

Structural  Composition  of  Steel.  —  Bearing  in  mind  that  hypo-eutectoid  steel  is 
composed  of  free  ferrite  and  pearlite  and  that  hyper-eutectoid  steel  consists  of  free 
cementite  and  pearlite,  and  knowing  the  proportion  of  carbon  in  pearlite  (0.85  per 
cent?)  and  in  cementite  (6.67  per  cent),  the  structural  composition  of  any  steel  may 
be  readily  calculated,  provided  we  know  the  percentage  of  carbon  it  contains. 

In  case  of  hypo-eutectoid  steel  we  have  the  two  following  equations  : 

(1)  F  +  P  =  100 

(2) 


100 

in  which  F  represents  the  percentage  of  free  ferrite  in  the  steel,  P  the  percentage  of 
pearlite,  E  the  percentage  of  carbon  in  pearlite,  and  C  the  percentage  of  carbon  in 
the  steel.  The  first  equation  expresses  the  fact  that  the  steel  is  composed  of  ferrite 
and  pearlite  and  the  second  equation  the  fact  that  all  the  carbon  in  the  steel  is  in- 


LESSON   V  —  MEDIUM   HIGH   AND   HIGH   CARBON   STEEL  9 

eluded  in  the  pearlite.  Assuming,  for  instance,  that  pearlite  contains  0.85  per  cent 
carbon  and  the  steel  0.50  per  cent  carbon,  the  resolution  of  these  two  equations  indi- 
cates that  steel  of  that  grade  has  the  following  structural  composition: 

F  =  per  cent  free  ferrite  =  41.8 
P  =  per  cent  pearlite  =  58.2 

In  case  of  hyper-eutectoid  steel  the  following  two  equations  may  be  written  : 

(1)  P  +  Cm  =  100 


100          100 

in  which  P  represents  the  percentage  of  pearlite,  Cm  the  percentage  of  free  cemen- 
tite,  E  the  percentage  of  carbon  in  pearlite,  C  the  percentage  of  carbon  in  the  steel. 
The  first  equation  expresses  the  fact  that  hyper-eutectoid  steel  is  composed  of  pear- 
lite  and  free  cementite  and  the  second  the  fact  that  the  carbon  in  the  steel  is  dis- 
tributed between  the  pearlite  and  the  free  cementite,  forming  E  per  cent  of  the 
pearlite  and  6.67  per  cent  of  the  cementite.  Assuming  the  value  of  E  to  be  0.85  and 
the  steel  to  contain  1.25  per  cent  carbon,  these  equations  give  for  a  steel  of  that  grade 

P  =  per  cent  pearlite  =  93 
Cm  =  per  cent  free  cementite  =  7 

Supposing  that  pearlite  or  eutectoid  steel  contains  0.85  per  cent  carbon,  since  the 
whole  of  that  carbon  is  present  in  the  cementite  plates  of  pearlite  and  since  cemen- 
tite contains  6.67  per  cent  carbon  (as  called  for  by  its  chemical  formula  FesC),  the 
percentage  of  cementite  in  pearlite  may  be  readily  calculated,  as  follows: 

'•  —  X  per  cent  cementite  =  0.85 
100 

i  on 

hence,  per  cent  cementite  =  0.85  x  -  =  12.74 

6.67 

and  per  cent  ferrite  =  100  -  12.74  =  87.26 

or  roughly  1  part  by  weight  of  cementite  to  6.6  parts  by  weight  of  ferrite. 

If  it  be  considered,  however,  (1)  that  the  exact  carbon  content  of  pearlite  is  not, 
and,  hardly  can  be,  known,  (2)  that  it  varies  somewhat  both  with  composition  and 
treatment,  and  (3)  that  in  commercial  steel  it  is  probably  not  far  from  0.85  per  cent, 
we  are  fully  warranted  to  assume,  for  the  sake  of  the  great  simplicity  it  introduces 
in  the  calculations,  that  pearlite  contains  exactly  1  part  by  weight  of  cementite  to 
7  parts  by  weight  of  ferrite,  which  would  be  the  case  if  the  eutectoid  point,  corre- 
sponded to  0.834  per  cent  carbon,  as  indicated  below: 

1  part  cementite  +  7  parts  ferrite  yields  8  parts  pearlite 
or  12.50  per  cent  cementite  +  87.50  per  cent  ferrite  =  100  per  cent  pearlite 

and  since  cementite  contains  6.67  per  cent  carbon,  12.50  per  cent  cementite  will  con- 
tain 6.67  X  .1250  =  0.834  per  cent  carbon.  Assuming  then  that  such  is  the  carbon 
content  of  eutectoid  steel,  so  that  1  part  of  cementite  gives  exactly  8  parts  by  weight 
of  pearlite  and,  noting  that  the  carbon  in  the  steel  produces  exactly  15  times  its  own 


10  LESSON  V  — MEDIUM   HIGH  AND  HIGH  CARBON  STEEL 

weight  of  cementite,1  the  calculation  of  the  structural  composition  of  any  steel  be- 
comes extremely  simple. 

In  case  of  hypo-eutectoid  steel  (steel  containing  less  than  0.834  per  cent  carbon) 
we  have 

per  cent  total  cementite  =  per  cent  total  carbon  x  15 
and  per  cent  pearlite  =  per  cent  total  cementite  x  8 

or,  more  simply, 

per  cent  pearlite  =  per  cent  carbon  x  120 
i.e.  P  =  120  C 

and,  of  course,  per  cent  ferrite  =  F  =  100  -  P. 

With  hyper-eutectoid  steel  (steel  containing  more  than  0.834  per  cent  carbon)  the 
figuring  is  as  follows: 

Since  8  parts  of  pearlite  contains  7  parts  of  ferrite  and  since  in  hyper-eutectoid 
steel  the  totality  of  the  ferrite  (total  ferrite)  is  included  in  the  pearlite  (there  being 
no  free  ferrite)  we  have 

per  cent  pearlite  =  P  =  f  total  ferrite 
or,  since  total  ferrite  =  100  —  total  cementite, 
P  =  f  (100  -  total  cementite) 

But  total  cementite  =  carbon  x  15,  hence 

P  =  f  (100  -  15  C) 

800  -  120  C 
or  P  =  - 

7 

and,  of  course,  free  cementite  =  Cm  =  100  —  P. 

Summing  up,  in  order  to  find  the  percentage  of  pearlite  in  hypo-eutectoid  steel  it 
will  suffice  to  multiply  its  carbon  content  by  120  (P  =  120  C),  the  balance  of  the  steel, 
consisting,  of  course,  of  free  ferrite  (F  =  100  —  P) ;  to  find  the  percentage  of  pearlite 
in  hyper-eutectoid  steel,  the  percentage  of  carbon  in  the  steel  should  be  substituted 

800  —  120  C 
for  C  in  the  formula:  P  =  -  -  and  the  balance  of  the  steel  will  be  made  -up 

of  free  cementite  (Cm  =  100  -  P). 

Taking,  for  instance,  a  steel  containing  0.50  per  cent  carbon.  Its  structural  com- 
position will  be: 

120  x  0.50  =  60  per  cent  pearlite,  and 

100  -  60  =  40  per  cent  ferrite 

If  a  steel  contains  1.25  per  cent  carbon  the  resulting  percentage  of  pearlite  will 

son  _  190  v  i  9^ 
be  -  -or  nearly  93  per  cent  and  the  free  cementite  (Cm),  100  -  93  =  7 

per  cent. 

1  This  follows  from  the  composition  of  FesC  indicated  by  the  atomic  weights  of  iron  and  carbon: 
(3  x  56)  Iron  +  12  Carbon  =  180  Fe3C 

180 

hence  one  part  carbon  produces  —  =  15  parts  FesC  or  cementite. 

12 


LESSON  V  — MEDIUM   HIGH  AND  HIGH  CARBON  STEEL  11 

In  these  lessons  it  will  be  assumed  for  the  sake  of  the  simplicity  it  introduces  that 
pearlite  contains  0.834  per  cent  carbon,  -that  is  exactly  1  part  by  weight  of  cementite 
to  7  parts  of  ferrite. 

Chemical  vs.  Structural  Composition.  —  Disregarding  for  the  present  the  existence 
of  impurities,  the  ultimate  analysis  of  steel  reveals  the  presence  of  so  much  carbon 
and  so  much  iron.  The  proximate  chemical  analysis  of  steel  reveals  (in  steel  slowly 
cooled  from  a  high  temperature)  the  presence  of  so  much  iron  and  so  much  carbide 
of  iron,  Fe3C.  In  a  similar  way  we  may  consider  two  different  structural  composi- 
tions, an  ultimate  and  a  proximate  one.  The  ultimate  structural  composition  reveals 
the  presence  of  so  much  total  ferrite  and  so  much  total  cementite,  while  the  proxi- 
mate structural  composition  informs  us  of  the  percentages  of  pearlite,  free  ferrite, 
and  free  cementite  in  the  steel.  It  will  be  evident  that  the  chemical  proximate  com- 
position is  identical  to  the  ultimate  structural  composition,  the  names  of  the  con- 
stituents only  being  different,  iron  and  carbide  in  the  first  case,  ferrite  and  cementite 
in  the  latter. 

These  various  compositions  are  tabulated  below: 

Constituents 

ultimate        Fe  C 

Chemical  Composition  .  _,  _, 

proximate     J?  e  b  e3C 

ultimate        total  ferrite    total  cementite 
Structural  Composition  .  ...  ..       .  ,  ... 

proximate     pearlite  tree  territe  free  cementite 

It  is  apparent  that  the  proximate  structural  composition  affords  more  valuable 
information  than  is  obtainable  through  the  other  three  kinds  of  analysis,  for  not  only 
does  it  indicate  the  chemical  nature  of  the  proximate  constituents  but  also  their 
structural  association  and  occurrence,  upon  which  depend,  to  a  very  great  extent,  the 
physical  properties  of  steel.  In  the  following  table  (page  12)  the  ultimate  chemical 
composition  as  well  as  the  structural  composition,  both  ultimate  and  proximate,  of 
steel  containing  from  0.1  to  2.0  per  cent  of  carbon,  have  been  calculated  for  each 
increase  of  carbon  of  0.1  per  cent.  Corrections  for  variations  of  0.01  per  cent  carbon 
can  readily  be  obtained  by  interpolation  and  are  indicated  in  a  second  table.  The 
values  given  for  the  proximate  compositions  are  based  upon  the  assumption  that 
pearlite  contains  0.834  per  cent  carbon. 

These  compositions  are  shown  also  diagrammatically  in  Figure  16  which  will  be 
readily  understood.  ABC  represents  the  free  ferrite  in  hypo-eutectoid  steel,  ACD  the 
pearlite  in  hypo-eutectoid  steel,  DCEF  the  pearlite  in  hyper-eutectoid  steel,  DFG  the 
free  cementite  in  hyper-eutectoid  steel,  ABEH  the  total  ferrite  in  any  steel,  AHG 
the  total  cementite  in  any  steel,  ACEH  the  pearlite-ferrite  in  any  steel,  and  AHFD 
the  pearlite-cementite  in  any  steel. 


12 


LESSON  V  — MEDIUM   HIGH  AND   HIGH  CARBON  STEEL 


CHEMICAL  COMPOSITION 

STRUCTURAL  COMPOSITION 

ULTIMATE 

ULTIMATE 

PROXIMATE 

C 

Fe 

Total  Cementite 

Total  Ferrite 

Pearlite 

Free  Ferrite 

Free  Cementite 

.1 

99.9 

1.5 

98.5 

12.0 

88.0 

_ 

.2 

99.8 

3.0 

97.0 

24.0 

76.0 

— 

.3 

99.7 

4.5 

95.5 

36.0 

64.0 

— 

.4 

99.6 

6.0 

94.0 

48.0 

52.0 

— 

.5 

99.5 

7.5 

92.5 

60.0 

40.0 

— 

.6 

99.4 

9.0 

91.0 

72.0 

28.0 

— 

.7 

99.3 

10.5 

89.5 

84.0 

16.0 

— 

.8 

99.2 

12.0 

88.0 

96.0 

4.0 

— 

.9 

99.1 

13.5 

86.5 

98.7 

— 

1.3 

1.0 

99.0 

15.0 

85.0 

97.0 

— 

3.0 

1.1 

98.9 

16.5 

83.5 

95.3 

— 

4.7 

1.2 

98.8 

18.0 

82.0 

93.6 

— 

6.4 

1.3 

98.7 

19.5 

80.5 

91.9 

— 

8.1 

1.4 

98.6 

21.0 

79.0 

90.2 

— 

9.8 

1.5 

98.5 

22.5 

77.5 

88.5 

— 

11.5 

1.6 

98.4 

24.0 

76.0 

86.8 

— 

13.2 

1.7 

98.3 

25.5 

74.5 

85.1 

— 

14.9 

1.8 

98.2 

27.0 

73.0 

83.4 

— 

16.6 

1.9 

98.1 

28.5 

71.5 

81.7 

— 

18.3 

2.0 

98.0 

30.0 

70.0 

80.0 

— 

20.0 

CARBON 

HYPO-EUTECTOID  STEEL 

HYPER-EUTECTOID  STEEL 

% 

Values  to  be  added  to  %  of  pearlite  and 
subtracted  from  %  cementite 

Values  to  be  subtracted  from  %  pearlite  and 
added  to  %  cementite 

0.01 

1.2 

0.17 

0.02 

2.4 

0.34 

0.03 

3.6 

0.51 

0.04 

4.8 

0.68 

0.05 

6.0 

0.85 

0.06 

7.2 

1.02 

0.07 

8.4 

1.19 

0.08 

9.6 

1.36 

0.09 

10.8 

1.53 

Micro-Test  for  Determination  of  Carbon  in  Steel.  —  Since  the  amount  of  pearlite 
in  steel  is  proportional  to  the  percentage  of  carbon  it  contains,  it  should  be  possible  to 
estimate  the  latter  with  a  fair  degree  of  accuracy  from  the  area  occupied  by  the 
pearlite.  After  a  little  experience  and  by  taking  the  necessary  precautions  it  will  be 
found  that,  in  the  case  of  decidedly  hypo-eutectoid  steels  at  least  (steels  containing 
say  less  than  0.60  per  cent  carbon),  results  are  obtained  fully  as  accurate  as  those  of 
the  colorimetric  method  and,  on  the  whole,  more  reliable,  since  the  possibility  of 
serious  errors  is  practically  eliminated.  By  the  micro-test,  for  instance,  a  steel  with 
0.25  per  cent  carbon  might  be  reported  as  containing  0.20  or  0.30  per  cent  of  that 


LESSON   V  — MEDIUM   HIGH   AND   HIGH   CARBON   STEEL 


13 


o  o 

ftj  n 


N  cy 


I 


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«0 


14  LESSON  V  — MEDIUM  HIGH  AND  HIGH  CARBON  STEEL 

element,  but  it  could  hardly  be  reported  as  containing  0.15  or  0.35  per  cent.  With 
chemical  methods  on  the  contrary,  even  with  the  combustion  method,  such  errors  are 
possible,  and  occasionally  occur,  through  mechanical  loss,  faulty  manipulations,  im- 
pure reagents,  mistakes  in  weighing  or  figuring,  etc.  Chemical  analysis  calls  for  the 
complete  destruction  of  the  anatomy  of  the  metal,  destroying  at  the  same  time  evi- 
dences of  serious  error;  micrographic  analysis,  on  the  contrary,  is  based  upon  the 
anatomy  itself  and  therefore  very  serious  errors  are  quite  impossible. 

In  order  to  yield  results  at  all  satisfactory,  however,  care  should  be  taken  that  all 
samples  be  first  annealed,  that  is  reheated  to  900  or  1000  deg.  C.  and  cooled  slowly, 
so  that  the  normal  amounts  of  pearlite  may  be  formed.  To  attempt  to  apply  the 
micro-test  to  forged  samples  for  instance  is  certain  to  lead  to  failure.  Nor  can  the 
test  be  applied  to  hypo-eutectoid  steel  containing  but  a  slight  amount  of  free  ferrite, 
for  instance  to  steel  with  from  0.60  to  0.80  per  cent  carbon,  because  of  the  difficulty 
of  estimating  accurately  the  area  occupied  by  so  small  a  proportion  of  the  constit- 
uent in  excess,  and  therefore  by  the  pearlite  itself.  In  the  case  of  hyper-eutectoid 
steel,  the  differences  between  the  contents  of  free  cementite  in  steels  of  materially 
different  carbon  contents  is  so  small  as  to  resist  accurate  determination.  For  instance, 
steels  respectively  with  1.10  and  1.40  per  cent  carbon  will  contain  95.3  and  90.2  per 
cent  pearlite,  a  difference  of  less  than  5  per  cent  in  their  contents  of  pearlite,  a  quan- 
tity too  small  to  be  estimated  with  satisfactory  accuracy  under  the  microscope. 

To  sum  up,  the  micro-test  for  the  determination  of  carbon  in  steel,  if  it  is  to  replace 
chemical  determinations,  should  be  applied  only  to  steels  containing  less  than  some 
0.60  per  cent  carbon  which  have  been  annealed  as  above  stated. 

The  author  has  found  the  following  method  to  yield  in  some  instances  satisfactory 
results:  the  sample  after  annealing  and  quick  polishing  and  etching  (a  few  small 
polishing  scratches  will  not  matter)  is  placed  under  the  microscope,  using  a  16  mm. 
objective  and  a  5X  eyepiece,  and  its  image  thrown  on  the  screen  of  the  camera.  In 
place  of  the  ordinary  camera  screen,  however,  another  screen  is  substituted  of  ground 
glass,  ruled  into  81  squares  (9x9),  so  that  every  square  covered  by  pearlite  evidently 
means  very  nearly  0.01  per  cent  of  carbon  in  the  steel  (exactly  0.01  per  cent  carbon 
if  we  assume  pearlite  to  contain  0.81  per  cent  carbon).  It  is  then  sufficient  to  esti- 
mate the  number  of  squares  occupied  by  pearlite  to  arrive  at  the  carbon  content  of 
the  steel.  The  results  may  be  checked  by  estimating  the  carbon  in  two  or  more  dif- 
ferent spots  and  reporting  the  average  if  the  agreement  is  sufficiently  close. 

Physical  Properties  of  the  Constituents  of  Steel.  —  It  will  now  be  timely  and 
profitable  to  inquire  into  the  physical  properties  of  the  three  constituents,  ferrite, 
cementite,  and  pearlite  of  which  steel  in  its  normal  condition  is  composed. 

It  will  be  evident  that  the  physical  properties  of  commercial  ferrite  must  resemble 
closely  those  of  wrought  iron  and  of  very  low  carbon  steel.  Ferrite,  therefore,  is  very 
soft,  very  ductile  and  relatively  weak,  having  a  ductility  corresponding  to  an  elonga- 
tion of  at  least  40  per  cent  and  a  tensile  strength  of  some  50,000  pounds  per  square 
inch.  It  is  magnetic,  has  a  high  electric  conductivity,  and  is  deprived  of  hardening 
power,  industrially  speaking  at  least,  since  carbonless  iron  cannot  be  materially 
hardened  by  rapid  cooling  from  a  high  temperature. 

The  properties  of  pearlite  are  evidently  those  of  eutectoid  steel  in  its  normal,  i.e. 
pearlitic  condition,  from  which  we  may  infer  that  pearlite  has  a  tenacity  of  some 
125,000  pounds  per  square  inch,  an  elongation  of  some  10  per  cent,  that  it  is  hard, 
and  for  reasons  later  to  be  explained,  that  it  possesses  maximum  hardening  power. 


LESSON  V  — MEDIUM   HIGH   AND   HIGH   CARBON  STEEL 


15 


With  the  exception  of  its  very  great  hardness  little  is  positively  known  as  to  the 
physical  properties  of  cementite.  It  may  be  assumed,  however,  that  so  hard  and 
brittle  a  substance  must  greatly  lack  tenacity.  Its  tensile  strength  probably  does 
not  exceed  5000  pounds  per  square  inch  and  may  be  considerably  less,  while  its 
ductility  must  be  practically  nil.  It  possesses  no  hardening  power. 

These  properties  of  the  constituents  of  steel  in  its  normal  condition  are  tabulated 
below : 


CONSTITUENTS 

TENSILE  STRENGTH 
LBS.  PER  SQ.  IN. 

ELONGATION 

%   IN  2   IN. 

HARDNESS 

•  . 

HARDENING  POWER 

Ferrite 

50,000  * 

40  ± 

Soft 

None 

Pearlite 

125,000  ± 

10  * 

Hard 

Maximum 

Cementite 

5000  (?) 

0 

Very  hard 

None 

Tenacity  of  Steel  vs.  its  Structural  Composition.  —  Knowing  the  physical  prop- 
erties of  the  three  constituents  of  steel,  it  should  be  possible  to  foretell  with  some 
degree  of  accuracy  the  physical  properties  of  any  steel  of  known  structural  com- 
position, on  the  reasonable  assumption  that  these  constituents  impart  to  the  steel 
their  own  physical  properties  in  a  degree  proportional  to  the  amounts  in  which  they 
are  present.  The  properties  of  steel  made  up  for  instance  of  50  per  cent  ferrite  and 
50  per  cent  pearlite  should  be  the  means  of  the  properties  of  ferrite  and  of  pearlite. 
Let  us  assume  such  reasoning  to  be  correct  and  let  us  apply  it  to  the  tensile  strength 
first  of  hypo-eutectoid  steel  and  then  of  hyper-eutectoid  steel. 

The  tensile  strength  (T)  of  any  hypo-eutectoid  steel  will  be  expressed  by  the  fol- 
lowing formula  in  function  of  its  structural  composition,  that  is  in  function  of  the 
percentages  of  ferrite  (F)  and  pearlite  (P)  which  it  contains: 

50,000  F  +  125,000  P 
1  = 

100 

in  which  50,000  represents  the  tensile  strength  of  ferrite  and  125,000  the  strength  of 
pearlite. 

Or  simplifying: 

T  =  500  F  +  1250  P 

or  again  in  terms  of  pearlite  alone,  since  F  =  100  -  P 

T  =  500  (100  -  P)  +  1250  P 
or  T  =  50,000  +  750  P 

or  finally  in  terms  of  carbon  since  P  =  120  C 

T  =  50,000  +  90,000  C. 

On  applying  this  simple  formula  to  steels  containing  respectively  0.10,  0.25,  and 
0.50  per  cent  carbon  we  find  for  these  metals  tensile  strengths  respectively  of  59,000, 
72,500,  and  95,000  pounds  per  square  inch.  These  values  agree  closely  with  our 
knowledge  of  the  average  tenacity  of  such  steels  when  in  a  pearlitic  condition,  and 


16  LESSON  V  — MEDIUM   HIGH   AND  HIGH   CARBON   STEEL 

prove  the  value  of  the  formula  derived  from  the  considerations  outlined  above  as  to 
the  relation  existing  between  the  physical  properties  of  steel  and  its  structural  com- 
position. It  should  be  borne  in  mind  that  in  working  out  this  formula  it  has  been 
assumed  that  pearlite  contains  0.834  per  cent  carbon. 

The  values  obtained  for  various  hypo-eutectoid  steel  should  be  accurate  only  for 
steel  in  what  has  been  termed  in  these  lessons  its  normal  condition,  that  is  steel 
which  has  been  forged,  reheated  to  a  high  temperature,  and  slowly  cooled.  It  should 
be  noted,  however,  as  later  explained,  that  steel  forged  and  finished  at  a  fairly  high 
temperature  are  practically  in  this  so-called  normal  condition,  so  that  the  formula 
may  be  used,  and  fair  results  expected,  to  calculate  the  tensile  strength  of  such  hot 
forged  steel.  If  the  steel  be  forged  until  its  temperature  is  quite  low  and,  especially, 
if  it  be  cold  worked,  it  is  well  known  that  its  tensile  strength  is  generally  increased. 
Neither  can  the  formula  be  used,  of  course,  in  the  case  of  hardened  steel  or  of  steel 
castings.  It  may,  however,  be  applied  to  steel  castings  which  have  been  properly 
annealed,  when  the  tensile  strength  may  be  brought  up  to  the  level  of  steel  forgings 
finished  fairly  hot  as  explained  in  another  lesson. 

Again  the  formula  is  of  value  only  in  case  of  commercial  steels  containing  the 
usual  proportions  of  impurities  especially  of  manganese.  It  applies  only  to  steels  in 
which  the  percentage  of  manganese  varies  roughly  with  the  carbon  content  from 
some  0.20  to  0.80  per  cent.  The  presence  of  a  larger  proportion  of  manganese  would 
increase  the  tenacity  materially. 

Passing  to  the  tensile  strength  of  hyper-eutectoid  steel,  our  ignorance  as  to  the 
tenacity  of  cementite  does  not  permit  the  writing  of  a  formula  with  the  same  degree 
of  confidence.  Let  us  assume,  tentatively,  however,  that  cementite  has  a  tensile 
strength  of  5000  pounds  per  square  inch  and  then  proceed  as  we  did  in  the  case  of 
hypo-eutectoid  steel. 

The  tensile  strength  of  any  hyper-eutectoid  steel  may  be  expressed  by  the  follow- 
ing formula  in  terms  of  the  percentages  of  pearlite  (P)  and  cementite  (Cm)  which  it 
contains : 

125,000  P  +  5000  Cm 

100 
or  simplifying 

T  =  1250  P  +  50  Cm 

or  in  terms  of  pearlite  only,  since  Cm  =  100  -  P, 

T  =  1250  P  +  100  (50  -  P) 
T  =  5000+  1150  P 

800  -  120  C 
or  since,  as  previously  shown,  P  =  -     

T=  5000+ 1150 

or  simplifying 

955,000  -  138,000  C 

or  approximately  T  =  136,000  -  20,000  C. 

Applying  this  formula  to  steels  containing  respectively  1.25  and  1.50  per  cent 
carbon,  we  find  for  their  respective  strength  111,000  and  106,000  per  square  inch, 


LESSON   V  — MEDIUM   HIGH   AND   HIGH   CARBON   STEEL  17 

which  are  fair  values  for  the  average  tenacity  of  pearlitic  steels  of  those  degrees  of 
carburization.1 

Steel  of  Maximum  Strength.  —  From  the  preceding  considerations  it  seems  evi- 
dent that  eutectoid  steel  must  possess  maximum  tensile  strength  since  the  influence 
of  the  presence  of  ever  so  small  an  amount  of  free  ferrite  in  hypo-eutectoid  steel  or 
of  free  cementite  in  hyper-eutectoid  steel  must  necessarily  be  a  weakening  one,  be- 
cause of  the  relative  weakness  of  free  ferrite  and  free  cementite  as  compared  to  the 
strength  of  pearlite.  By  most  writers,  on  the  other  hand,  steel  of  maximum  tenacity 
is  often  stated  to  contain  in  the  vicinity  of  1  per  cent  carbon,  that  is  to  be  slightly 
hyper-eutectoid. 

It  is  not  clear,  however,  that  the  results  upon  which  the  statement  is  based  were 
obtained  in  testing  steel  in  its  pearlitic  condition.  On  the  contrary  it  seems  probable 
that  a  large  number  of  the  steels  tested  were  in  a  sorbitic  rather  than  in  a  pearlitic 
condition  because  of  relatively  quick  cooling  through  the  critical  range  as  explained 
in  a  subsequent  lesson.  And  while  it  appears  that  pearlitic  steel  must  have  its  maxi- 
mum tenacity  when  composed  entirely  of  pearlite,  it  may  well  be  that  when  in  a  sor- 
bitic condition  maximum  strength  corresponds  to  a  higher  degree  of  carburization, 
i.e.  1  per  cent,  because  sorbite  may  contain  and  indeed  often  does  contain  more  carbon 
than  pearlite.  Indeed  the  cases  on  record  show  that  when  the  steels  were  made  pear- 
litic through  very  slow  cooling  maxium  tenacity  corresponds  closely  to  the  eutectoid 
composition.  Arnold,  for  instance,  tested  a  series  of  very  pure  carbon  steel  and  after 
slow  cooling  in  the  furnace  from  1000  deg.  C  he  found  a  very  sharp  maximum  in  the 
tenacity  corresponding  to  0.89  per  cent  carbon.  On  cooling  these  same  steels  in  air, 
on  the  contrary,  and  therefore  making  them  sorbitic,  maximum  tenacity  corresponded 
to  1.20  per  cent  carbon.  Harbord  likewise  ascertained  the  tenacity  of  very  pure 
steels  and  found  after  slow  cooling  (in  the  furnace)  from  900  deg.  C  that  the  maxi- 
mum tenacity  corresponded  to  0.947  per  cent  carbon. 

Ductility  of  Steel  vs.  Its  Structural  Composition.  —  From  the  known  ductility, 
as  expressed  by  its  elongation  under  tension,  of  ferrite  and  the  known  elongation  of 

1  Empirical  formulas  have  often  been  suggested  to  express  the  relation  between  the  tenacity  of 
steel  and  its  carbon  content.  Deshayes  proposed  for  unannealed  steel 

T  =  30.09  +  18.05  C  +  36.11  C2 
Thurston  (minimum  values)  for  unannealed  steel 

T  =  42.32  +  49.37 
and  for  annealed  steel 

T  =  35.27  +  42.32 
Bauschinger  for  Bessemer  steel 

T  =  43.64  (1  +  C2) 
Weyrauch  (minimum  values) 

T  =  44.17  (1  +C) 
Salom  (average  values) 

T  =  31.74  +  70.53  C. 

The  above  formulas  express  the  tenacity  in  kilograms  per  square  millimeter.    Campbell,  for  acid 
open  hearth  steel,  gives 

T  =  40.000  +  1000  C  +  1000  P  +  xMn  +  R 
and  for  basic  open  hearth  steel 

T  =  41.500  +  770  C  +  1000  P  +  yMn  +  R 

in  which  x  and  y  are  values  given  in  a  table  and  dependent  upon  the  percentage  of  manganese  and 
of  carbon  present.    R  is  a  variable  to  allow  heat  treatment. 


18 


LESSON  V  — MEDIUM   HIGH  AND  HIGH   CARBON   STEEL 


pearlite,  respectively  40  and  10  per  cent  in  two  inches,  it  should  be  possible  to  work 
out  a  formula  expressing  the  ductility  of  any  hypo-eutectoid  steel  in  the  annealed 
(pearlitic)  condition.  In  terms  of  ferrite  and  pearlite  the  ductility  should  be 


D= 


40  F  +  10  P 
100 


or  simplifying 

D  =  .4  F  +  .1  P 

or  in  terms  of  pearlite  alone  since  F  =  100  -  P 

D  =  .4  (100  -  P)  +  .1  P  =  40  -  .3  P 

and  since  P  =  120  C,  the  ductility  in  terms  of  carbon  will  be 

D  =  40  -  36  C 

Pearlitic  steels,  for  instance,  containing  0.25  and  0.50  per  cent  carbon  should  have 
elongations  respectively  of  31  and  22  per  cent.1 


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iagram  showing  the  relation  between  the  tenacity  and  ductility  of  annealed  (pearlitic 
steels  and  the  carbon  content. 

1  It  is  interesting  to  compare  this  formula  with  some  others  that  have  been  proposed.     Howe 
gives  for  the  elongation  in  8  inches  of  steel  under  0.50  per  cent  carbon 

D  =  33  -  60  (C2  +  0.1) 
and  for  steel  between  0.50  and  1.00  per  cent  carbon: 

D  =  12  -  11.9  V^C- 0.5 
Deshayes  for  the  elongation  in  8  inches,  gives 

D  =  42  -  56  C 
and  for  the  elongation  in  4  inches 

D  =  35  -  30  C 

These  formulas  give  lower  values  for  the  elongation  of  steel  than  the  author's  formula,  but  all  indi- 
cations point  to  the  fact  that  they  refer  to  steel  in  a  rather  sorbitic  condition  and,  therefore,  more 
tenacious  and  less  ductile,  whereas  the  formula  here  suggested  is  for  truly  pearlitic  steel  only. 


I 


LESSON   V  — MEDIUM   HIGH   AND   HIGH    CARBON    STEEL  19 

Diagram  Showing  the  Relation  Between  the  Tenacity  and  Ductility  of  Steel  and 
Its  Carbon  Content.  —  By  plotting  the  formulas  suggested  in  this  lesson  to  express 
the  relation  between  the  carbon  content  of  steel  and  its  tenacity  and  ductility  the 
curves  of  Figure  17  are  obtained.  To  the  tenacity  and  ductility  curves  a  third  curve 
has  been  added  showing  the  variation  of  the  amount  of  pearlite  with  the  carbon 
content. 

Experiments 

The  student  should  procure  five  samples  of  forged  steel  of  good  commercial  qual- 
ity, containing  respectively  about  0.30,  0.50,  0.85,  1.25,  and  1.50  per  cent  carbon. 
These  should  be  heated  to  900  or  1000  deg.  C.  and  slowly  cooled  from  that  tempera- 
ture. Specimens  should  be  cut  from  the  treated  samples  and  prepared  for  micro- 
scopical examination  in  the  usual  way. 

Etching.  —  Any  one  of  the  three  methods  described  in  Lessons  III  and  IV  may 
be  applied  with  satisfactory  results.  The  student  is  advised  to  use  the  method  which, 
in  connection  with  the  experiments  of  the  two  previous  lessons,  he  has  found  to  give 
the  best  results. 

Examinations.  —  All  samples  should  be  carefully  examined,  firstly  with  low  power 
objectives  and  secondly  with  high  power  objectives.  Observation  under  low  power 
should  in  all  cases  reveal  clearly  and  sharply  the  pearlite  and  ferrite  areas  in  hypo- 
eutectoid  steel,  the  pearlite  and  cementite  areas  in  hyper-eutectoid  steel.  Examina- 
tion under  high  power,  300  diameters  or  more,  should  satisfactorily  reveal  the 
structure  of  the  pearlite.  In  the  case  of  the  eutectoid  steel,  low  power  observation 
will  reveal  but  an  indistinct  structure,  because  of  the  absence  of  any  free  ferrite  or 
cementite  and  of  the  fineness  of  the  pearlite  structure. 

Etching  with  Sodium  Picrate.  —  The  hyper-eutectoid  steels  should  be  rubbed  on 
the  rouge  block  or  disk  so  as  to  efface  the  pattern  produced  by  the  etching  and  treated 
with  a  boiling  solution  of  sodium  picrate  in  an  excess  of  sodium  hydroxide  in  order  to 
color  darkly  the  free  cementite.  The  instructions  given  in  the  lesson  for  this  opera- 
tion should  be  followed. 

Photomicrography.  —  All  samples  should  be  photographed  both  with  low  and 
high  power  objectives.  In  taking  low  power  photomicrographs  the  directions  given 
in  Lesson  IV  should  be  followed. 

For  the  taking  of  high  power  photographs  a  4  mm.  (%  in.)  objective  and  a  5X 
(2  in.)  or  if  needed  a  10X  (1  in.)  eyepiece  are  recommended.  With  the  camera  screen 
some  15  inches  from  the  eyepiece  a  magnification  of  325  diameters  will  be  obtained 
in  case  of  the  5X  eyepiece  and  of  650  diameters  with  the  10X  eyepiece. 

The  needed  manipulations  for  the  taking  of  high  power  photomicrographs  are  the 
same  as  to  their  nature  as  those  required  for  taking  low  power  photographs  but  they 
call  for  much  greater  accuracy.  The  adjustment  of  the  source  of  light  and  of  all 
parts  used  in  condensing  the  light,  including  the  vertical  illuminator  should  be  done 
with  the  greatest  possible  care  and  delicacy  while  the  focusing  of  the  image  on  the 
camera  screen  could  hardly  receive  too  much  attention.  The  student  is  urged  not  to 
be  discouraged  if  his  first  attempts  at  taking  high  power  photomicrographs  are  fail- 
ures. Patience,  perseverance,  and  experience  will  eventually  lead  to  the  mastery  of 
the  manipulations  required  for  successful  high  power  photomicrography  of  metal 
sections. 


20  LESSON  V  — MEDIUM   HIGH   AND  HIGH   CARBON   STEEL 

Examination 

I.     Describe  the  structure  of  hypo-eutectoid,  of  eutectoid,  and  of  hyper-eutectoid 
steel. 

II.  Assuming  pearlite  to  contain  0.834  per  cent  carbon,  what  will  be  the  proximate 
structural  composition  of  steels  containing  respectively  0.12,  0.27,  0.56,  and 
1.15  per  cent  carbon?  What  will  be  their  ultimate  structural  composition? 

III.  Accepting  as  correct  the  formulas  given  in  these  lessons  what  will  be  the  tensile 

strength  of  the  steels  mentioned  in  Question  II. 

IV.  Mention  any  difficulty  encountered  in  conducting  the  experiments  of  this  lesson. 


LESSON  VI 

IMPURITIES   IN  STEEL 

Metallic  Impurities.  —  Commercial  grades  of  steel  always  contain,  besides  carbon, 
varying  amounts  of  silicon,  phosphorus,  sulphur,  and  manganese,  often  an  appreciable 
proportion  of  copper  and  traces  at  least  of  many  other  metals  and  metalloids.  These 
may  be  called  the  metallic  impurities. 

Non-metallic  or  Oxidized  Impurities.  —  Non-metallic  or  oxidized  impurities, 
chiefly  oxides  and  silicates  of  iron  and  manganese,  are  also  frequently  found  in  steel, 
principally  through  the  retention  by  the  metal  of  some  of  the  slag  produced  during 
the  refining  operation.  Hibbard  has  recently  suggested  the  name  of  "sonims"  for 
this  class  of  impurities. 

Metallic  vs.  Non-metallic  Impurities.  —  There  is  a  sharp  distinction  between  the 
behavior  of  metallic  and  non-metallic  impurities,  the  former  forming  true  alloys  with 
the  contaminated  metal,  the  latter  being  merely  inclusions,  their  union  with  the 
metal  being  purely  mechanical. 

Gaseous  Impurities.  —  Steel  always  contains  some  gases,  apparently  held  in  solu- 
tion and  called  "occluded"  gases,  chiefly  hydrogen,  nitrogen,  and  carbon  monoxide 
(CO). 

Impurities  vs.  Physical  Properties  of  Steel.  —  It  is  well  known  that  surprisingly 
small  proportions  of  some  of  the  metallic  impurities  just  mentioned  have  a  very 
marked  influence  upon  the  physical  properties  of  steel.  Some  0.2  per  cent  phosphorus, 
for  instance,  renders  many  grades  of  steel  so  brittle  as  to  unfit  them  for  most  com- 
mercial uses.  And  as  it  is  logical  to  suppose  that  there  exists  a  very  close  relation 
between  the  structure  of  a  metal  and  its  physical  characteristics,  we  naturally  ex- 
pect to  find  important  structural  changes  corresponding  to  marked  alterations  of 
physical  properties.  We  should  expect,  for  instance,  the  structure  of  a  high  phos- 
phorus, brittle  steel  to  be  quite  different  from  the  structure  of  a  low  phosphorus, 
tough  steel  of  otherwise  identical  composition.  In  the  present  state  of  metallography 
the  microscope  does  not  always  reveal  such  differences  of  structures  as  we  are  led  to 
look  for.  We  may  reasonably  anticipate,  however,  that,  as  the  science  progresses, 
structural  differences  will  be  detected  of  a  magnitude  fairly  in  keeping  with  the  deep 
changes  of  physical  properties  brought  about  by  slight  changes  of  chemical  composi- 
tion. Indeed  in  recent  years  material  advance  has  been  made  in  this  direction  and 
the  influence  of  the  usual  impurities  upon  the  properties  of  steel  has  been  on  the 
whole  satisfactorily  accounted  for  by  metallographic  methods  as  will  be  apparent 
from  the  description  which  follows. 

Silicon  in  Steel.  —  All  grades  of  steel  contain  a  trace  at  least  of  silicon  (Si)  and 
occasionally  as  much  as  0.5  per  cent,  and  even  more,  most  grades  containing  between 
0.05  and  0.3  per  cent. 

1 


2  LESSON  VI  —  IMPURITIES   IN  STEEL 

When  present  in  such  small  proportion  silicon  is  entirely  dissolved  in  the  iron 
with  which  it  forms  a  solid  solution.  It  is  probable,  however,  that  it  is  not  held  in 
solution  by  the  iron  in  its  elementary  condition,  Si,  but  rather  as  a  silicide  of  iron, 
FeSi.  Since  the  atomic  weight  of  iron  is  56  and  that  of  silicon  28  it  will  be  evident 
that  28  parts  by  weight  or  silicon  produces  56  +  28  or  84  parts  by  weight  of  FeSi,  or 

/84        \ 

that  silicon  produces  exactly  3  times  its  own  weight  of  FeSi  (  —  =  3  I.    For  instance 

\28        / 

0.1  per  cent  silicon  in  the  steel  will  give  rise  to  the  formation  of  0.3  per  cent  of  FeSi 
and  this  small  amount  of  iron  silicide  will  be  held  in  solid  solution  by  the  iron.  The 
ferrite  of  commercial  steel,  therefore,  always  contains  a  small  amount  of  silicon  in 
the  form  of  an  iron  silicide,  and  let  it  be  borne  in  mind  that  this  applies  to  the  ferrite 
forming  part  of  the  pearlite  of  all  slowly  cooled  steels  as  well  as  to  the  free  ferrite  of 
hypo-eutectoid  steel. 

It  has  been  stated  in  another  lesson  that  when  an  impurity  forms  a  solid  solution 
with  the  contaminated  metal,  changes  of  crystalline  forms  are  not  generally  ob- 
served. This  is  true  in  the  present  case  for  there  is  apparently  no  structural  difference 
between  a  steel  with  some  0.3  or  0.4  per  cent  silicon  and  a  steel  nearly  free  from  that 
element  but  otherwise  of  identical  composition.  The  presence  of  silicon  in  steel  can- 
not as  yet  be  satisfactorily  detected,  even  qualitatively,  by  metallographic  methods, 
although  we  have  the  unquestionably  accurate  statement  of  Le  Chatelier  that  silicon 
causes  ferrite  to  etch  more  slowly. 

In  view  of  the  similarity  of  structure  between  steel  containing  much  silicon  (i.e. 
several  tenths  of  1  per  cent)  and  steel  practically  free  from  it,  we  should  expect  that 
the  presence  of  a  small  amount  of  silicon  cannot  affect  materially  the  properties  of 
steel,  and  this  we  know  to  be  the  case. 

Phosphorus  in  Steel.  —  Steel  of  satisfactory  quality  contains  from  a  trace  to  0.1 
per  cent  of  phosphorus  (P).  As  in  the  case  of  silicon  this  small  amount  of  phosphorus 
is  held  in  solid  solution  by  the  iron,  not,  however,  in  the  elementary  state,  P,  but  as 
the  phosphide  of  iron  FesP.  The  atomic  weight  of  iron  being  56,  that  of  phosphorus 
31,  and  the  phosphide  containing  three  atoms  of  iron  for  each  atom  of  phosphorus,  it 
will  be  obvious  that  31  parts  by  weight  of  phosphorus  will  form  3  x  56  +  31  or  199 
parts  of  the  phosphide  Fe3P,  or  roughly,  1  part  by  weight  of  phosphorus  will  give 
rise  to  the  formation  of  6  parts  of  phosphide.  For  instance,  the  presence  in  steel  of 
0.05  per  cent  phosphorus  results  in  the  formation  of  0.3  per  cent  of  Fe3P  held  in  solid 
solution  by  the  ferrite,  this  being  true  of  the  ferrite  included  in  the  pearlite  of  all 
slowly  cooled  steel  as  well  as  of  the  free  ferrite  of  hypo-eutectoid  steel. 

While  phosphorus  in  common  with  other  metallic  impurities  forming  solid  solu- 
tions does  not  alter  the  crystalline  form  of  steel,  it  is  generally  believed  to  have  a 
marked  tendency  to  enlarge  the  grains  of  the  metal,  which  tendency  would  account 
for  the  well-known  brittleness  imparted  to  steel  by  phosphorus  when  present  in  ex- 
cess of  0.1  per  cent.  The  brittleness  caused  by  a  large  grain  will  be  considered  fur- 
ther in  another  lesson. 

Except  for  this  enlargement  of  the  grains,  microscopical  examination  does  not  re- 
veal the  presence  of  the  usually  small  percentages  of  phosphorus  occurring  in  steel, 
although  it  is  said  by  some  writers  that  phosphorus  as  well  as  manganese  causes  ferrite 
to  etch  darker. 

Sulphur  in  Steel.  —  Steel  of  satisfactory  commercial  quality  may  contain  from  a 
mere  trace  to  some  0.1  per  cent  sulphur,  generally  between  0.01  and  0.05  per  cent. 


LESSON   VI  —  IMPURITIES   IN   STEEL 


It  is  universally  known  that  manganese  and  sulphur  have  very  great  reciprocal  af- 
finity so  that  when  brought  together  at  a  high  temperature  they  combine  chemically 
with  each  other  to  form  the  sulphide  of  manganese,  MnS.  This  is  what  happens  in 
steel  which  always  contains  manganese  as  well  as  sulphur.  From  the  atomic  weight 
of  manganese,  55,  and  that  of  sulphur,  32,  it  will  be  seen  that  32  parts  by  weight  of 
sulphur  produces  87  parts  of  MnS,  or  approximately  2^  parts  of  sulphide  for  each 
part  of  sulphur.  Steel  with  0.05  per  cent  sulphur,  for  instance,  will  contain  about 
0.125  per  cent  of  MnS,  provided,  of  course,  there  is  enough  manganese  present  to 
satisfy  the  sulphur  which  must  necessarily  be  so  in  properly  made  steel. 

The  existence  of  the  sulphide  of  manganese,  MnS,  in  steel  has  been  conclusively 
proven.  In  steel  castings  it  occurs  as  rounded  areas  the  color  ef-  which  is  generally 
described  as  pale  or  dove  gray  or  slate  color.  In  forgings  it  occurs  in  elongated 
particles,  bands,  or  strings  of  the  same  tint,  running  parallel  to  the  direction  of  the 
forging  or  rolling  (Figs.  1  and  2).  -^ ^^ 


Fig.  1. — Steel.    Carbon  0.46  per  cent.  Manganese     Fig.  2. — Steel.     Hypo-eutectoid.     Manganese 
1.07  per  cent.    Sulphur  0.54  per  cent.    Forged,         sulphide    in    ferrite    areas.      Magnified    300 
reheated   to    1200  deg.   C.   and   cooled  in  air.         diameters.      (Levy.) 
Magnified  460  diameters.    Right  half  section  is 
longitudinal  (direction  of  rolling),  the  left  half  is 
transverse.    (Arnold.) 

According  to  Le  Chatelier  MnS  has  a  melting-point  superior  even  to  that  of  iron, 
solidifying,  therefore,  first,  and  the  bulk  of  it  rising  to  the  top  of  the  bath  or  ingot, 
the  manganese  is  in  this  way  helpful  in  removing  sulphur  from  steel.  Some  writers, 
however,  question  this  higher  melting-point  of  MnS.  Levy  reports  that  the  melting- 
point  of  pure  MnS  is  probably  not  far  from  1400  deg.  C.,  and,  therefore,  below  the 
melting-point  of  hypo-eutectoid  steel  at  least,  while  the  presence  of  some  FeS  would 
lower  materially  its  melting-point.  It  appears  probable  that  the  solidification  of  the 
sulphide  must  follow  and  not  precede  that  of  the  iron. 

This  view  seems  to  be  supported  by  the  location  of  the  sulphide  particles  at  the 
boundaries  of  the  pearlite  grains  of  eutectoid  steel,  in  the  free  ferrite  of  hypo-eutec- 
toid steel  or  in  the  free  cementite  of  hyper-eutectoid  steel.  What  MnS  is  retained  by 
the  solid  steel,  since  it  occurs  as  shown  in  the  shape  of  small  individual  grains  or 


4  LESSON  VI  —  IMPURITIES   IN  STEEL 

elongated  particles,  can  only  injure  the  metal  through  breaking  up  its  continuity 
and,  in  view  of  the  very  small  amount  of  sulphur,  and,  therefore,  of  MnS,  present  in 
steel  of  good  quality,  it  is  evident  that  this  breaking  up  and  its  action  upon  the  prop- 
erties must  be  very  slight.  This  is  in  agreement  with  the  known  fact  that  a  small 
amount  of  sulphur  in  steel  containing  also  the  proper  amount  of  manganese  has  no 
appreciably  injurious  effect. 

Seeing  that  steel  seldom  contains  much  more  than  some  0.05  per  cent  sulphur, 
hence  more  than  0.125  per  cent  MnS,  it  is  not  to  be  expected  that  this  compound 
will  generally  be  detected  in  polished  and  etched  steel  sections.  Indeed  whenever 
detected  it  points  to  a  segregation  of  the  sulphide  together  with  other  impurities 
(ghost  lines)  as  described  later. 

In  case  sulphur  occurs  in  excess  over  the  amount  needed  to  form  the  sulphide 
MnS  with  the  manganese  present  in  the  steel,  the  excess  sulphur,  that  is  the  sulphur 
left  over  after  satisfying  the  manganese,  combines  with  some  of  the  iron,  forming  the 
iron  sulphide  FeS.  It  should  be  noted  at  once,  however,  that  it  requires  less  than 
2  parts  by  weight  of  manganese  (atomic  weight  55)  to  combine  with  1  part  of  sulphur 
(atomic  weight  32).  In  other  words  if  the  steel  contains  twice  as  much  manganese 
as  it  does  sulphur,  this  should  theoretically  be  enough  to  convert  the  whole  of  the 
sulphur  into  the  sulphide  MnS.  As  it  is  very  seldom  indeed  that  steel  does  not  con- 
tain a  much  larger  proportion  of  manganese  than  that  compared  to  its  sulphur  con- 
tent, the  occurrence  of  free  FeS  in  steel  should  be  very  rare.  It  is  not  to  be  expected 
in  metal  of  good  quality,  its  presence  pointing  to  a  very  abnormal  composition, 
namely  high  sulphur  content  and  very  low  percentage  of  manganese.  According  to 
Levy,  however,  MnS  and  FeS  are  readily  soluble  in  each  other  in  the  solid  state,  MnS 
being  capable  of  holding  as  much  as  50  per  cent  of  FeS  in  solid  solution.  According 
to  this  writer  MnS  is  seldom  free  from  FeS  even  when  the  steel  contains  considerable 
manganese,  the  mass  action  exerted  by  the  presence  of  so  large  a  proportion  of  iron 
preventing  the  manganese  from  taking  hold  of  the  totality  of  the  sulphur  in  spite  of 
its  greater  affinity  for  it.  MnS  nearly  free  from  FeS  appears  quite  dark,  while  its 
color  becomes  lighter  and  more  yellowish  as  the  proportion  of  FeS  increases.  Levy 
notes  also  that  in  high  carbon  steel  the  MnS  areas  are  generally  colored  darker  than 
in  low  carbon  steel,  indicating  greater  freedom  from  FeS,  apparently  owing  to  the 
fact  that  in  high  carbon  steel  the  mass  action  exerted  by  iron  is  not  so  great  since  it 
contains  less  iron. 

The  sulphide  FeS  exhibits  a  marked  tendency  to  form  continuous  envelopes  or 
membranes  surrounding  each  grain  of  pearlite  (Fig.  3),  and  probably  consisting  of  a 
eutectic  alloy  of  iron  and  iron  sulphide  (the  composition  of  the  eutectic  is  apparently : 
FeS  85  per  cent,  Fe  15  per  cent).  These  membranes  being  weak  and  brittle  impart 
weakness  and  brittleness  to  the  steel.  The  well-known  red-shortness  caused  by  sul- 
phur in  the  absence  of  a  sufficient  amount  of  manganese  (to  form  MnS)  is  probably 
due  to  the  low  melting-point  (950  deg.  C.  according  to  some  writers)  of  this  iron- 
iron  sulphide  eutectic.  At  a  high  temperature  the  melting  of  this  eutectic  destroys 
the  cohesion  between  the  grains  of  the  metal  resulting  in  cracks  being  developed  dur- 
ing the  process  of  forging  or  rolling,  and  in  extreme  cases  in  the  metal  actually  break- 
ing into  several  pieces.  The  presence  of  a  large  amount  of  FeS  in  some  Bessemer  steel 
at  the  end  of  the  blow,  before  the  addition  of  manganese,  is  undoubtedly  largely  re- 
sponsible for  the  marked  red-shortness  of  the  metal  at  this  stage  of  the  operation. 

Under  the  microscope  FeS  appears  yellow  or  pale  brown.     Tests  showing  the 


LESSON   VI  —  IMPURITIES   IN   STEEL  5 

presence  of  sulphur  in  the  constituents  described  above  as  sulphides  may  be  con- 
ducted as  follows,  provided  they  occur  in  sufficiently  large  particles :  A  sheet  of  silver 
bromide  (photographic)  paper  should  be  pressed  upon  the  polished  section  and 
moistened  with  sulphuric  acid  when  the  sulphur  present  will  be  evolved  as  H2S  (sul- 
phuretted hydrogen)  and  will  darken  the  paper.  Another  method  (Law)  consists  in 
covering  the  section  with  a  coating  of  gelatine  containing  an  acid  solution  of  lead  or 
cadmium  salt;  the  acid  decomposes  the  sulphide  forming  H2S  which  produces  a  deep 
brown  or  yellow  stain  of  lead  or  cadmium  sulphide,  PbS  or  CdS. 

Manganese  in  Steel.  —  It  has  been  seen  that  manganese  combines  readily  with 
sulphur  and  that  the  resulting  manganese  sulphide,  MnS,  can  be  detected  in  polished 
steel  sections  as  a  pale  or  dove  gray  constituent  assuming  the  shape  of  rounded  areas 
in  castings  and  of  bands  or  threads  in  forgings.  Manganese  silicate  is  also  occasionally 


Fig.  3.  —  Red  short  steel.    Magnified  300  diameters.    Sulphur 
0.54  per  cent.    Unetched.    Network  of  FeS.    (Ziegler.) 

found  in  steel  as  later  explained  and  may  sometimes  be  mistaken  for  MnS.  Satis- 
factory tests  for  the  distinction  of  these  two  constituents  will  be  described. 

When  manganese  occurs  in  excess  over  the  amount  required  to  form  MnS  with 
the  totality  of  the  sulphur  present,  as  is  almost  universally  the  case,  the  manganese 
in  excess  combines  with  some  of  the  carbon  to  form  the  carbide  of  manganese,  Mn3C, 
and  this  carbide  is  found  associated  with  the  iron  carbide,  Fe3C,  in  cementite.  The 
comentite  of  commercial  steel,  therefore,  is  seldom  a  pure  iron  carbide,  containing, 
on  the  contrary  varying  amounts  of  Mn3C.  Since  iron  and  manganese  have  practi- 
cally the  same  atomic  weight,  however  (55  and  56  respectively),  it  remains  practically 
true  that  carbon  forms  15  times  its  own  weight  of  cementite,  even  when  the  latter 
contains  a  large  proportion  of  MnsC. 

There  is  no  metallographic  test  by  which  cementite  free  from  manganese  can  be 
distinguished  from  cementite  rich  in  MnaC. 

Some  authors  mention  the  possible  presence  of  the  manganese  silicide,  MnSi,  in 
steel,  while  solid  solution  between  manganese  and  iron  is  frequently  referred  to. 
While  manganese  and  iron  (ferrite)  undoubtedly  form  solid  solutions,  it  does  not 
seem  likely  that  these  are  produced  when  manganese  is  present  in  small  proportion, 


6  LESSON  VI  —  IMPURITIES  IN  STEEL 

say  not  over  1  per  cent.  In  that  case  it  seems  more  probable  that  manganese  is  found 
in  the  two  forms  described  above,  (1)  as  a  manganese  sulphide  MnS,  containing  prac- 
tically the  totality  of  the  sulphur  in  steel  of  good  quality,  that  is,  containing  not  over 
0.05  per  cent  sulphur  and  not  less  than  0.25  per  cent  manganese,  and  (2)  as  the  man- 
ganese carbide  Mn3C,  associated  with  Fe3C  in  cementite. 

Chemical  vs.  Structural  Composition.  —  Knowing  the  probable  chemical  forms  of 
the  five  metallic  impurities  always  present  in  steel,  carbon,  silicon,  phosphorus,  sul- 
phur, and  manganese,  as  well  as  their  structural  associations,  it  will  be  interesting 
and  profitable  to  consider  accordingly  the  proximate  chemical  composition  as  well  as 
the  ultimate  and  proximate  structural  compositions  of  a  steel  of  known  ultimate 
chemical  composition.  Let  us  assume  a  steel  of  the  following  ultimate  chemical 
composition: 

C        0.50  per  cent 

Mn    0.80    "      " 

S        0.05    "      " 

P        0.04    "      " 

Si       0.10    "      " 
Fe  (by  diff.)   98.51    "      " 

100.00 

Bearing  in  mind  the  atomic  weights  of  these  elements  (Fe,  56;  C,  12;  Mn,  55;  S,  32; 
P,  31;  Si,  28)  and  the  formulas  of  the  chemical  compounds  formed  (MnS,  FeSi,  Fe3P, 
Mn3C,  Fe3C),  it  will  be  readily  seen  that: 

(1)  0.05  per  cent  S  will  give  rise  to  the  formation  of  0.13  per  cent  MnS. 

(2)  0.13  per  cent  MnS  contains  about  0.08  per  cent  Mn. 

(3)  This  leaves  0.80  -  0.08  =  0.72  per  cent  manganese  in  excess  to  combine  with  C. 

(4)  0.72  per  cent  Mn  will  form  0.77  per  cent  MnsC. 

(5)  0.77  per  cent  Mn3C  contains  about  0.05  per  cent  carbon. 

(6)  This  leaves  0.50  —  0.05  =  0.45  carbon  to  combine  with  iron. 

(7)  0.45  per  cent  carbon  results  in  the  formation  of  6.75  per  cent  of  Fe3C. 

(8)  0.04  per  cent  of  P  corresponds  to  about  0.25  per  cent  of  Fe3P. 

(9)  0.10  per  cent  Si  gives  0.30  per  cent  FeSi. 

The  proximate  chemical  composition  of  the  steel  considered  will  be 


Fe3C 

6.75  per 

cent 

Mn3C 

0.77    " 

u 

Fe3P 

0.25    " 

a 

FeSi 

0.30    " 

u 

MnS 

0.13    " 

ti 

Fe  (by  diff.) 

91.80    " 

n 

100.00 

As  to  the  ultimate  structural  composition  of  the  steel,  we  know  that  the  cemen- 
tite contains  the  Fe3C  and  the  Mn3C  hence  we  have  6.75  +  0.77  =  7.52  per  cent 
cementite.  In  pure  steel  the  percentage  of  cementite  would  have  been  0.50  X  15  = 
7.50  per  cent.  The  slight  difference  between  the  two  numbers  is  due  to  the  presence 
of  manganese  in  the  commercial  steel,  and  to  a  slight  difference  between  the  atomic 


LESSON   VI  —  IMPURITIES   IN   STEEL  7 

weights  of  manganese  and  that  of  iron  (55  compared  to  56),  a  difference  so  slight 
that  for  all  practical  purposes  we  may  assume  that  in  commercial  steels  as  well  as  in 
pure  steel  the  percentage  of  carbon  multiplied  by  15  gives  the  amount  of  cementite 
formed.  The  total  ferrite  present  in  this  steel  contains  all  the  free  iron,  as  well  as  the 
small  proportions  of  Fe3P  and  FeSi  present,  hence  this  steel  contains  91.80  +  0.25  + 
0.30  =  92.35  total  ferrite.  In  pure  steel  the  proportion  of  total  ferrite  would  have 
been  100  —.7.50  or  92.50  per  cent.  The  difference  between  the  two  values  is  evi- 
dently due  to  the  presence  of  a  trifle  greater  amount  of  cementite,  and  to  the  pres- 
ence of  0.13  per  cent  MnS. 

The  ultimate  structural  composition  of  the  steel  under  consideration  is,  therefore: 

Cementite     7.52 

Total  ferrite   92.35 

MnS  0.13 


100.00 

Finally  its  proximate  structural  composition  will  be,  since  the  pearlite  of  hypo- 
eutectoid  steel  contains  8  times  the  weight  of  total  cementite  (assuming  the  eutec- 
toid  carbon  point  to  be  0.834  per  cent) : 

Pearlite  7.52  x  8  =  60.16 

Free  ferrite  (by  diff.)  39.71 

MnS     .13 


100.00 

Ignoring  the  presence  of  impurities  the  quick  method  described  in  Lesson  V 
would  have  given  pearlite  60  per  cent,  ferrite  40  per  cent,  i.e.  values  which  may  be 
considered  identical  for  any  practical  purposes.  It  follows  from  this  that  in  calcu- 
lating the  structural  composition  of  any  carbon  steel  of  ordinary  commercial  quality 
the  presence  of  the  impurities  need  not  be  considered;  the  steel  may  be  treated  as  if 
it  was  made  exclusively  of  iron  and  carbon. 

The  relation  between  chemical  and  structural  compositions,  both  ultimate  and 
proximate,  is  further  shown  in  the  following  table. 


CHEMICAL  COMPOSITION 


STRUCTURAL  COMPOSITION 


ULTIMATE 

PROXIMATE 

ULTIMATE                                                  PROXIMATE 

Fe(by  diff 
Si 

)  98.51 
0.10 

Fe(by  diff.)  91.80  ' 
FeSi               0.30 

[  Free  Ferrite                    39.71  Free 
Total  Ferrite  92.35  <                                                      Ferrite 

P 
C 
Mn 

S 

0.04 
0.50 
0.80 
0.05 

FesP              0.25 
Fe3C              6.75  ' 
MnsC             0.77  j 
MnS              0.13 

(  Pearlite  Ferrite  52.64  j  60-16%  pear. 
Cementite        7  52                                         j                    nte 

MnS                 0.13    0.13 

100.00 

100.00 

100.00                                           100.00 

8 


LESSON  VI  —  IMPURITIES   IN   STEEL 


Non-Metallic  or  Oxidized  Impurities.  —  As  already  mentioned  steel  not  infre- 
quently contains  small  amounts  of  non-metallic  or  oxidized  impurities,  chiefly  iron 
and  manganese  oxides  and  silicates,  derived  mainly  from  the  retention  by  the  metal 


Fig.  4.  —  Manganese  sulphide  (light  constituent)  and  manganese  silicate 
in  steel.    Magnified  1000  diameters.    (Law.) 


Fig.  5.  —  Manganese  sulphide  (light  constituent)   and  iron  silicate  in 
mild  steel.    Unetched.    Magnified  1000  diameters.    (Law.) 

in  the  shape  of  minute  particles  of  some  of  the  slag  formed  during  the  process  of 
manufacture.  Their  mode  of  occurrence  is  very  different  from  that  of  the  metallic 
impurities  just  examined  (with  the  exception  of  MnS  which  behaves  more  like  a  non- 
metallic  than  like  a  metallic  impurity).  These  oxidized  impurities  do  not  alloy  with 


LESSON   VI  —  IMPURITIES   IN   STEEL  9 

the  metal;  their  association  with  it  remains  a  purely  mechanical  one,  like  small  peb- 
bles in  a  mass  of  clay.  These  oxides  and  silicates  commonly  occur  as  rounded  or 
slightly  elongated  particles  and  can  generally  be  detected  in  the  polished  section 
before  etching. 

Manganese  silicate,  probably  2MnO.3SiC>2,  and  manganese  sulphide,  MnS,  which 
occasionally  occur  together,  have  a  somewhat  similar  appearance,  care  being  re- 
quired in  order  to  differentiate  between  them,  although  the  former  is  as  a  rule  de- 
cidedly darker  (Fig.  4).  Stead  recommends  the  placing  of  a  drop  of  sulphuric  acid  on 
the  polished  specimen,  when  H2S  gas  will  be  evolved  where  MnS  is  present,  particles  of 
silicates  of  manganese,  on  the  contrary,  evolving  no  gas.  The  dissolving  of  MnS 
also  leaves  pits.  Stead  also  advises  heat  tinting  as  the  best  means  of  distinguishing 
between  the  sulphide  and  the  silicate,  the  heating  to  be  continued  until  the  specimen 
has  assumed  a  light  brown  coloration,  when  the  MnS  remaining  bright  can  be  sharply 
differentiated  from  the  silicate. 

According  to  Levy,  sulphide  and  silicate  of  manganese  are  readily  soluble  when 

• 


w 


v; 


^•H     '          '  : 

Fig.  6.  —  Ghost  lines  in  low  carbon  steel.    Magnified 
95  diameters.     (Boylston.) 

molten  but  on  solidifying  the  sulphide  crystallizes  in  well-marked  dendritic  forms, 
the  resulting  mixture  of  sulphide  and  silicate  (Fig.  4)  resembling  slag  inclusions  (see 
Lesson  III,  Fig.  5). 

Manganese  sulphide  and  iron  silicate  may  also  occur  in  close  vicinity,  the  latter 
constituent  being  darker  and  frequently  broken  in  many  irregular  fragments  by  the 
working  of  the  metal  (see  Fig.  5). 

At  the  end  of  the  refining  operation  by  which  steel  is  produced,  especially  towards 
the  latter  part  of  the  Bessemer  blow,  a  considerable  amount  of  iron  oxide  is  formed 
and  in  spite  of  the  steps  taken  for  removing  it  from  the  bath  (addition  of  manganese, 
etc.)  some  of  it,  occasionally  quite  a  little,  is  retained  by  the  metal,  when  it  is  a  source 
of  red-shortness  besides  having  other  detrimental  effects.  This  iron  oxide  generally 
occurs  as  small  dark  points  readily  detected  in  the  polished  section  before  etching. 

For  further  treatment  of  polished  sections  with  a  view  of  identifying  oxidized  im- 
purities, the  student  is  referred  to  Lesson  III  where  the  constitution  of  slag  in  iron 
has  been  treated  at  some  length. 

Gaseous  Impurities.  —  It  has  not  been  possible  so  far  to  detect  the  presence  of 
occluded  gases  in  steel  by  means  of  metallographic  methods.  While  the  problem 


10 


LESSON  VI  —  IMPURITIES   IN   STEEL 


seems  a  very  difficult  one  to  solve,  the  statement  that  it  can  never  be  solved  would 
not  be  justified  for  the  discovery  of  some  metallographic  treatment  by  which  a  metal 
rich  in  certain  gases  may  be  distinguished  from  a  similar  metal  free  from  them  is  well 
within  the  limits  of  reasonable  expectation. 


Fig.  7.  —  Ghost  lines  in  low  carbon  steel.    Magnified  10  diameters.     (Law.) 


Fig.  8.  —  Ghost  line.s  in  low  carbon  steel.    Magnified  200  diameters.     (Law.) 

Segregation  of  Impurities.  —  Ghosts.  —  The  very  small  proportions  of  impurities 
generally  found  in  steel  of  good  quality  have  little,  if  any,  injurious  effect  upon  its 
most  important  and  useful  physical  properties,  so  long  as  they  remain  uniformly 
distributed  throughout  the  metallic  mass,  i.e.  so  long  as  the  steel  is  chemically  homo- 
geneous. These  impurities,  on  the  contrary,  may  become  extremely  injurious  when 


LESSON   VI  —  IMPURITIES   IN   STEEL  11 

they  show  a  tendency  to  "segregate,"  i.e.  to  collect  in  certain  portion  or  portions  of 
steel  castings  and  forgings,  when  the  segregated  portions  may  contain  so  large  an 
amount  of  impurities  as  to  have  their  useful  properties  utterly  destroyed.  Segre- 
gated metal  is  generally  brittle,  weak,  and  hard. 

Under  the  microscope  a  metal  suffering  from  this  segregation  of  impurities  gen- 
erally is  found  to  contain  bands  of  varying  widths  and  lengths,  technically  known  as 
"ghosts"  or  "ghost  lines,"  in  which  the  presence  of  abnormally  large  proportions  of 
MnS,  phosphorus,  and  carbon  can  generally  be  detected  by  the  ordinary  metallo- 
graphic  tests,  these  (S,  P,  and  C)  being  the  three  impurities  .showing  the  greatest  ten- 
dency to  segregate.  Photomicrographs  of  ghost  lines  are  shown  in  Figures  6,  7,  8,  and  9. 
Ghost  lines  etch  more  rapidly  than  the  surrounding  metal  therefore  appearing  darker 


- 


t 


Fig.  9.  —  Ghost  lines  in  low  carbon  steel.     Magnified  2000  diameters. 
Manganese  sulphid  and  pearlite  particles.     (Law.) 

after  etching  even  to  the  naked  eye.    These  lines  can  generally  be  detected  before 
etching  because  of  the  manganese  sulphide  which  they  contain. 

Bannister  mentions  two  kinds  of  ghost  lines,  (1)  those  showing  marked  segrega- 
gation  of  C,  S,  and  P,  and  considerable  Si  and  Mn  and  (2)  those  containing  little  Si 
and  Mn.  Houghton,  on  the  other  hand,  refers  to  ghost  lines  containing  S  and  P  but 
no  carbon. 

Experiments 

High  vs.  Low  Phosphorus  Steel.  —  The  student  should  procure  two  samples  of 
forged  steel  containing  preferably  from  0.30  to  0.50  per  cent  carbon  and  of  nearly 
identical  composition,  except  as  to  phosphorus  content  which  should  be  high  in  one 
sample  (if  possible  considerably  more  than  0.1  per  cent),  and  low  in  the  other  (not 
over  0.05  per  cent).  These  samples  should  be  heat  treated  in  the  usual  way  so  that 
they  may  assume  their  normal  structure  and  a  specimen  prepared  from  each  sample 
for  microscopical  examination  etching  them  with  picric  or  nitric  acid  in  alcohol  or 
with  concentrated  nitric  acid  according  to  individual  preference.  Upon  being  ex- 


12  LESSON  VI  —  IMPURITIES  IN  STEEL 

amined  under  the  microscope  the  larger  grain  of  the  high  phosphorus  steel  should  be 
apparent. 

Photograph  each  sample,  using  such  magnification  as  will  best  bring  out  the  fea- 
ture to  be  illustrated,  namely  difference  in  grain  size. 

High  Sulphur  Steel.  —  A  sample  of  steel  casting  and  a  sample  of  steel  forging 
both  containing  if  possible  considerably  more  than  0.10  per  cent  sulphur  should  be 
obtained.  These  need  not  be  heat  treated  but  may  at  once  be  polished  and  exam- 
ined, first  before  etching  and  then  after  etching,  for  the  detection  of  sulphide  areas, 
as  described  in  the  lesson.  If  the  sulphide  areas  are  of  sufficient  size  the  chemical 
test  described  should  be  applied  for  the  detection  of  sulphur. 

The  samples  should  be  photographed  with  a  view  of  bringing  out  sharply  the 
sulphide  flaws. 

Oxidized  Bessemer  Metal.  —  A  sample  of  Bessemer  metal  preferably  high  in  sul- 
phur should  be  procured  if  possible,  taken  at  the  end  of  the  blow  and  before  the  ad- 
dition of  manganese  or  any  other  recarburizer.  A  specimen  of  suitable  size  should 
be  polished  and  examined  under  the  microscope,  both  before  and  after  etching.  The 
metal  should  contain  both  sulphide  of  iron,  FeS,  and  iron  oxide  the  appearance  of 
which  has  been  described  in  this  lesson.  The  specimen  should  be  photographed  so 
as  to  show  these  two  impurities. 

Segregated  Steel.  —  If  a  sample  of  segregated  steel  can  be  obtained  it  should  be 
prepared,  examined,  and  photographed  in  order  to  reveal  the  presence  of  ghost  lines. 


Examination 

I.    Describe  briefly  the  appearance  under  the  microscope  of  the  following  impuri- 
ties: Si,  S,  Mn,  P,  iron  oxide,  and  manganese  silicate. 

II.     Explain  the  meaning  of  "ghost"  lines. 
III.    A  steel  has  the  following  ultimate,  chemical  compostion: 

C         0.60  per  cent 
Mn      0.75    "      " 
S          0.04    "      " 
P          0.06    "      " 
Si         0.15    "      " 

Fe(bydiff.)  98.40   "      " 

100.00 

What  will  be  (a)  its  proximate  chemical  composition,  (6)  its  ultimate  structural 
composition,  and  (c)  its  proximate  structural  composition? 


LESSON   VII 

THE  THERMAL  CRITICAL  POINTS  OF  STEEL 
THEIR  OCCURRENCE 

The  structure  of  steel  described  in  the  preceding  lessons,  i.e.  its  normal  structure, 
is  greatly  affected  by  the  treatment  or  treatments,  both  mechanical  and  thermal,  to 
which  the  metal  may  be  subjected  during  the  process  of  manufacture  of  finished 
objects.  It  is  to  the  close  relation  existing  between  the  treatment  and  the  structure 
on  the  one  hand,  and  between  the  structure  and  the  physical  properties  of  the  metal 
on  the  other,  that  metallography  owes  its  industrial  importance.  It  is  essential, 
therefore,  that  the  student  should  have  a  clear  understanding  of  these  relations.  As 
a  preparation  to  this  important  study,  however,  it  will  be  necessary  to  describe  a 
phenomenon  of  the  greatest  moment  in  the  treatment  of  steel,  namely,  the  occurrence 
of  spontaneous  absorptions  or  evolutions  of  heat  during  the  heating  or  cooling  of  the 
metal.  These  are  generally  termed  the  "thermal"  critical  points  or  simply  "critical 
points,"  also  "retardations,"  "transformation"  points,  and  "critical  temperatures." 

Point  of  Recalescence.  —  If  a  piece  of  steel  containing  some  0.60  per  cent  carbon 
be  heated  to  a  high  temperature,  say  to  1000  deg.  C.,  and  allowed  to  cool  slowly 
from  that  temperature,  and  if  its  rate  of  cooling  be  carefully  ascertained,  conveniently 
by  means  of  a  Le  Chatelier  pyrometer,  it  is  found  that  the  cooling  proceeds  at  first  at 
a  nearly  uniformly  retarded  rate.  If,  for  instance,  it  requires  10  seconds  for  the  metal 
to  cool  through  the  first  five  degrees  (from  1000  to  995  deg.),  and  12  seconds  to  cool 
through  the  next  five  degrees  (995  to  990  deg.),  it  will  require  some  14  seconds  for 
the  next  five  degrees,  16  seconds  for  the  following  five,  and  so  on,  the  cooling  through 
each  range  of  five  degrees  being  a  little  slower  than  the  preceding  cooling  of  five 
degrees.  All  cooling  bodies,  whatever  their  nature,  generally  follow  this  law.  The 
plotting  of  time  and  temperature  as  coordinates  yields  smooth  curves,  sometimes 
approaching  straight  lines  (see  curve  B,  Fig.  9). 

In  the  case  of  the  steel  we  are  now  considering,  when  a  certain  temperature  is 
reached,  in  the  majority  of  cases  some  650  to  700  deg.  C.,  a  most  interesting  and 
significant  phenomenon  takes  place;  the  cooling  of  the  metal  is  momentarily  arrested, 
the  pyrometer,  for  a  certain  length  of  time,  failing  to  record  any  further  fall  of  tem- 
perature. Indeed,  when  the  circumstances  are  favorable,  the  temperature  of  the 
cooling  mass  actually  rises;  the  metal  becomes  visibly  hotter;  it  "recalesces,"  hence 
the  name  of  "  recalcscence "  given  to  this  thermal  critical  point.  If  the  experiment 
be  conducted  in  a  dark  room,  this  recalescence  or  spontaneous  glow  of  the  steel  is 
plainly  visible.  After  a  while  the  metal  resumes  its  normal  rate  of  cooling  which  is 
then  continued  down  to  atmospheric  temperature. 

It  is  evident  that  at  this  critical  point  the  surrounding  atmosphere  does  not  cease 
to  abstract  heat  from  the  piece  of  steel,  and,  since  its  temperature  nevertheless 

1 


2  LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 

remains  stationary  or  even  rises,  it  must  be  that  heat  is  here  spontaneously  gene- 
rated within  the  metal  in  amount  sufficient  to  make  up,  or  more  than  make  up,  for 
the  heat  lost  by  radiation  and  conductivity. 

In  heating,  as  might  be  expected,  the  reverse  phenomenon  takes  place :  an  absorp- 
tion of  heat  causing  a  retardation  in  the  rise  of  the  temperature,  or  even  a  momentary 
stop,  the  pyrometer  failing  for  a  few  moments  to  record  any  further  increase  of 
temperature  or  recording  only  an  abnormally  low  increase,  although  heat  continues 
to  be  applied  to  the  steel  at  the  same  speed.  Actual  lowering  of  the  temperature  of 
the  steel  is  not  generally  observed  at  this  critical  point  on  heating,  i.e.  the  steel  does 
not  grow  perceptibly  colder. 

Notation.  —  Osmond,  who  was  the  first  to  determine  accurately  the  position  and 
magnitude  of  the  point  of  recalescence  and  who  is  the  discoverer  of  the  upper  critical 
points  soon  to  be  described,  adopted  Tschernoff's  previous  notations,  and  designated 
the  critical  points  by  the  letter  A.1  To  distinguish  critical  points  on  cooling  from 
those  occurring  on  heating  the  former  are  called  Ar  (from  the  French  refroidissement, 
meaning  cooling)  and  the  latter  Ac  (from  the  French  chauffage,  heating).  To  dis- 
tinguish further  between  the  point  of  recalescence  and  its  reversal  on  heating  on  the 
one  hand,  and  critical  points  occurring  at  higher  temperatures  on  the  other,  the  nota- 
tions Ari  and  Aci  are  used  for  the  recalescence  point  and  its  reversal,  and  Ar2,  Ac2, 
Ar3,  Acs  for  the  two  upper  reversible  critical  points  soon  to  be  described.  The  nota- 
tions AI,  A2,  A3  are  frequently  used  when  the  points  and  their  reversals  are  consid- 
ered collectively.  By  the  notation  AI,  for  instance,  is  meant  the  point  of  recalescence 
Ari  and  its  reversal  Aci.  These  notations  will  be  used  in  these  lessons. 

Brinell,  in  his  important  work  on  the  heat  treatment  of  steel,  used  the  letter  V  for 
the  point  of  recalescence  and  W  for  its  reversal  on  heating.  These  symbols,  however, 
are  now  very  seldom  used. 

The  expression  "point  of  recalescence"  is  frequently  used  indifferently  for  the 
point  on  cooling,  where  heat  is  evolved  causing  a  recalescence  of  the  metal,  and  for 
the  reverse  phenomenon  on  heating,  at  Aci,  where,  of  course  instead  of  a  recalescence 
taking  place,  an  absorption  of  heat  occurs  causing  the  metal  to  lose  heat.  It  is  ob- 
vious that  the  term  recalescence  should  not  be  applied  to  the  point  Aci.  The  point 
of  recalescence  is  also  called  sometimes  recalescent  point  and,  seldom,  Gore's  phe- 
nomenon (see  Historical  Sketch  at  end  of  lesson).  The  point  Aci  has  been  called 
point  of  "  decalescence "  by  some  writers  and  one  of  them  at  least  refers  to  it  as 
the  "calescence"  point. 

Critical  Range.  —  Transformation  Range.  —  When  the  various  critical  points  oc- 
curring in  steel  are  considered  collectively  the  range  of  temperature  they  cover  is 
frequently  called  the  critical  range,  or,  more  seldom,  but  very  appropriately,  the 
transformation  range.  It  will  soon  be  shown  that  the  critical  range  may  include  one, 
two,  or  three  critical  points.  The  meaning  of  the  expressions  "critical  range  on  heat- 
ing" and  "critical  range  on  cooling"  is  obvious. 

Positions  of  Ari  and  Aci.  —  The  critical  points  Art  and  Aci  do  not  occur  at  exactly 
the  same  temperature,  Aci  being  generally  situated  some  25  to  50  deg.  higher 
than  Ari.  When  the  point  Ari,  for  instance,  is  found  at  690  deg.  C.,  the  point  API 
will  generally  occur  somewhere  between  715  and  740  deg.  It  does  not  follow,  how- 
ever, that  these  two  points  are  not  the  opposite  phases  of  the  same  phenomenon. 

1  The  point  A  of  Tschernoff  indicated  the  temperature  at  which  steel  suddenly  acquires  harden- 
ing properties  on  heating  or  loses  them  on  cooling. 


LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL  3 

The  fact  that  the  critical  point  on  cooling  lags  behind  the  point  on  heating  and  vice 
versa,  is  evidently  a  case  of  hysteresis  so  often  observed  in  physical  phenomena  and 
which  implies  a  resistance  of  certain  bodies  to  undergo  a  certain  transformation,  when 
theoretically  the  transformation  is  due,  the  delayed  transformation  finally  taking 
place  with  added  violence.  This  was  vividly  depicted  by  Howe  some  twenty  years 
ago  in  the  case  of  iron.  He  wrote:  "Just  as  we  can  cool  water  below  its  freezing- 
point  without  completely  freezing  it,  thereby  rapidly  increasing  the  strength  with 
which  the  water  tends  to  freeze,  so  by  a  relatively  rapid  cooling  we  can  carry  the 
metal  considerably  below  Ari  without  giving  the  An  change  time  to  proceed  far, 
strengthening  the  while  the  tendency  toward  this  change,  which  keeps  kindling  more 


gap  due  fo 
Hysferes/s 


Fig.  1.  —  Diagram  showing  reversible  critical  point. 


and  more  till  it  bursts  into  a  blaze,  with  such  evolution  of  heat  as  actually  to  reca- 
lesce,  to  raise  the  temperature  of  the  metal  by  some  10  deg.,  in  spite  of  the  continued 
abstraction  of  heat  by  the  continued  cooling  of  the  furnace." 

The  slower  the  heating  and  cooling  the  nearer  will  the  two  points  approach  each 
other,  so  that  with  infinitely  slow  cooling  and  heating  they  would  undoubtedly  occur 
at  exactly  the  same  temperature.  If  there  remained  any  doubt  as  to  the  points  Aci 
and  Ari  representing  the  opposite  phases  of  the  same  phenomenon,  i.e.  of  A  being 
a  reversible  point,  it  would  suffice  to  dispel  it  to  consider  the  fact  that  in  order  to 
induce  the  retardation  Art  the  steel  must  first  be  heated  past  the  point  Aci;  and  re- 
ciprocally the  retardation  Aci  cannot  take  place  unless  the  metal  has  first  been  cooled 
to  a  point  below  Ari.  To  illustrate :  the  melting  of  ice  and  the  freezing  of  water  are 
undoubtedly  the  opposite  phases  of  the  same  phenomenon,  each  one  undoes  the 
work  of  the  other,  and  in  order  to  freeze  the  water  we  must  first  melt  the  ice  and 
likewise  to  melt  the  ice  the  water  must  first  be  frozen;  one  change  cannot  be  induced 
unless  the  opposite  one  has  last  taken  place.  Indeed  it  is  possible  through  very  slow 


4  LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 

and  undisturbed  cooling  to  lower  the  temperature  of  water  below  its  freezing-point 
before  it  starts  freezing,  a  clear  instance  of  hysteresis,  although  in  this  case  called 
"surfusion,"  and  when  freezing  takes  place  the  temperature  of  the  water  rises  to  its 
normal  freezing-point,  a  clear  case  of  recalescence  although  deprived  of  glow. 

The  diagram  shown  in  Figure  1  illustrates  further  this  reversibility  of  the  point 
AI.  Let  two  parallel  lines  represent  the  phases  Ac  and  Ar  of  the  critical  point.  Let 
condition  A  represent  the  state  of  the  metal  stable  above  Ac  and  condition  B  the 
state  of  the  metal  stable  below  Ar.  The  gap  between  Ac  and  Ar  is  due  to  hysteresis. 
MN  is  the  temperature  at  which  both  the  Ac  point  and  the  Ar  point  would  occur  if 
there  was  no  hysteresis  as,  for  instance,  if  the  metal  could  be  heated  and  cooled  in- 
finitely slowly.  Assuming  the  metal  to  be  in  condition  B  at  a,  below  Ar,  on  heating 
it  from  a  to  b  above  Ac  on  reaching  the  Ac  point  at  x  it  passes  from  the  condition  B 
to  the  condition  A  with  absorption  of  heat  causing  a  retardation  in  the  heating;  on 
cooling  from  b  to  c,  that  is  to  a  temperature  below  Ac  but  above  Ar,  the  condition  A 
is  retained  so  that  upon  heating  from  c  to  d  no  transformation  can  take  place  at  x' 
on  passing  through  the  Ac  point  and  therefore  no  critical  point  observed.  If  the 
metal  be  cooled  from  d  to  e,  however,  on  passing  through  Ar  at  y  it  changes  from 
condition  A  to  condition  B  with  evolution  of  heat,  causing  a  retardation  in  the  rate 
of  cooling;  if  it  now  be  heated  again  from  e  to  /  above  Ac,  a  critical  point  will  be  ob- 
served at  x"  since  the  metal  now  in  condition  B  will  pass  to  condition  A. 

It  will  be  evident  that  between  Ar  and  Ac  the  metal  may  be  in  condition  A  or 
condition  B  depending  upon  whether  it  was  last  cooled  from  above  Ac  or  heated 
from  below  Ar. 

Speed  of  Cooling  and  Heating  vs.  Position  of  AI.  —  It  has  been  seen  that  the 
faster  the  cooling  the  lower  is  the  position  of  the  point  Ari  and  the  faster  the  heating 
the  higher  the  point  Aci,  that  is,  the  faster  the  cooling  and  heating  the  greater  the 
gap  between  the  opposite  phases  Ari  and  Aci  of  the  reversible  point  AI. 

The  cooling  of  a  piece  of  steel  may  be  so  rapid,  as  in  quenching,  as  to  prevent 
altogether  the  retardation  Ari  from  taking  place,  because  a  low  temperature  is  so 
quickly  reached  that  the  rigidity  of  the  metal  prevents  the  transformation  of  which 
An  is  a  manifestation.  In  other  words  time  and  a  certain  amount  of  plasticity  are 
required  for  the  transformation  Ari  to  occur,  and  in  quenching,  time  is  denied  when 
the  metal  is  sufficiently  plastic  (i.e.  at  a  red  heat),  while  when  time  is  given  (i.e.  after 
quenching)  the  metal  has  lost  its  plasticity.  It  remains  untransformed  or  but  par- 
tially transformed.  It  will  be  shown  in  another  lesson  that  this  suppression  of  the 
point  Ari  is  probably  the  cause  of  the  hardening  of  carbon  steel  by  sudden  cooling.1 

Le  Chatelier  rightly  reminds  us  that  the  speed  of  the  transformations  occurring 
at  the  critical  points  of  steel  follows  the  general  laws  which  govern  the  speed  of  all 
chemical  phenomena.  In  other  words  that  the  speed  of  the  transformation  is  the 
greater  (1)  the  higher  the  absolute  temperature  and  (2)  the  wider  the  range  between 
the  actual  temperature  and  the  temperature  of  equilibrium,  that  is  the  temperature 
at  which  the  transformation  is  due.  Above  the  critical  temperature  both  influences 
act  in  the  same  direction  and  the  speed  of  transformation  increases  without  limit. 
Below  the  critical  temperature  these  influences  act  in  opposite  directions  necessarily 
giving  rise  to  the  existence  of  a  maximum  speed.  According  to  Le  Chatelier  this 
notion  of  variable  speeds  of  transformation  accounts  for  all  the  peculiarities  of  the 

1  It  will  be  explained  later  that  some  writers  have  doubted  the  suppression  of  the  transforma- 
tions on  rapid  cooling  and  have  suggested  another  explanation  of  the  burdening  of  steel. 


LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL  5 

hardening  treatment.  On  heating  it  is  hardly  possible  to  raise  the  temperature  of 
transformation  more  than  100  deg.  C.  through  very  rapid  heating,  while  during  cool- 
ing the  speed  reaches  its  maximum  at  about  600  deg.  C.,  is  very  feeble  below  200,  and 
nearly  null  at  atmospheric  temperature. 

Chemical  Composition  vs.  Position  of  AI.  —  Generally  speaking  impurities  have  a 
tendency  to  lower  the  position  of  Aci  and  Ari,  some  of  them  decidedly.  Osmond,  for 
instance,  indicates  the  position  of  An  in  a  steel  containing  1  per  cent  of  Mn  as  685 
deg.  whereas  with  4  per  cent  of  manganese  the  same  point  was  lowered  to  590  deg. 
It  is  conceivable  that  further  increase  of  that  element  must  lower  still  more  the  crit- 
ical point,  so  that  finally  it  may  be  lowered  below  atmospheric^  temperature,  being 
apparently  eliminated.  It  will  be  shown  in  another  lesson  that  this  is  precisely  what 
occurs  in  the  cases  of  manganese  steel  and  high  nickel  steel,  containing  respectively 
some  13  per  cent  of  manganese  or  some  25  per  cent  of  nickel.  These  steels  exhibit  no 
retardation  on  cooling  from  a  high  temperature  to  atmospheric  temperature.  When 
cooled  to  lower  temperatures,  however,  by  immersing  them  in  freezing  mixtures,  or, 
if  need  be,  in  liquid  air,  the  retardations  may  again  occur.  In  the  case  of  commercial 
steel  of  good  quality  the  proportion  of  impurities,  with  the  possible  exception  of  man- 
ganese, varies  within  relatively  very  narrow  limits,  so  that  no  great  variation  should 
be  expected  in  the  position  of  the  critical  point  AI.  Neither  is  it  clear  that  the  amount 
of  carbon  present  in  steel  has  a  marked  effect  upon  the  position  of  the  point  AI,  al- 
though some  writers  state  that  the  point  is  lifted  as  the  carbon  increases.  The  point 
An  almost  invariably  occurs  somewhere  between  650  and  700  deg.  C.  and  its  reversal 
Aci  25  to  50  deg.  higher.  Both  Ari  and  Aci  would  probably  occur  at  about  710  deg. 
could  the  cooling  and  heating  be  infinitely  slow. 

Upper  Critical  Points.  —  The  existence  of  upper  critical  points,  that  is  of  thermal 
retardations  occurring  at  temperatures  higher  than  that  of  the  recalescence  point,  has 
already  been  alluded  to.  These  points  were  discovered  by  Osmond  and  their  dis- 
covery ushered  in  a  new  epoch  in  the  scientific  study  of  iron  and  steel.  To  describe 
these  points  it  is  advisable  to  consider  first  the  thermal  retardations  occurring  in 
cooling  and  heating  carbonless  iron  and  then  similar  retardations  exhibited  by  steel 
containing  increasing  amounts  of  carbon. 

Thermal  Critical  Points  in  Pure  Iron.  —  On  cooling  from  a  high  temperature,  say 
1000  deg.  C.,  a  piece  of  the  purest  iron  obtainable  and  ascertaining  its  rate  of  cooling 
as  previously  explained,  the  metal  is  found  to  cool  normally,  i.e.  at  a  uniformly  re- 
tarded rate,  until  a  temperature  of  some  900  to  850  deg.  C.  is  reached  when  a  marked 
retardation  is  observed  in  the  rate  of  cooling,  indicating  a  spontaneous  evolution  of 
heat,  in  this  case,  however,  insufficient  to  cause  an  actual  rise  of  temperature,  i.e.  a 
recalescence  of  the  metal.  The  cooling  then  resumes,  or  nearly  resumes,  a  normal 
rate  of  cooling,  until  at  about  750  deg.  C.  a  second  evolution  of  heat  takes  place 
causing  another  retardation  in  the  rate  of  cooling,  not  so  marked,  however,  nor  so 
sharply  defined  as  the  first  one.  The  metal  then  cools  normally  or  quite  so  to  atmos- 
pheric temperature.  We  have  thus  detected  two  unmistakeable  spontaneous  evolu- 
tions of  heat  in  the  cooling  of  pure  iron.  The  corresponding  critical  points  are  called 
Ar3  and  Ar2,  the  latter  symbol  indicating  the  lower  point.  It  should  be  noted  that 
the  recalescence  point  which  should  occur  at  some  675  deg.  is  here  absent.  Carbon- 
less iron  has  no  point  of  recalescence. 

These  two  upper  points  like  the  point  of  recalescence  are  reversible  critical  points, 
i.e.  on  heating  the  opposite  phases  of  the  transformations  (whatever  those  trans- 


6  LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 

formations  may  be)  take  place  with  absorption  of  heat,  causing  a  retardation  in  the 
rate  of  heating  and  the  corresponding  points  being  designated  by  the  symbols  Ac3 
and  Ac2.  The  point  Ac3  occurs  at  a  temperature  some  20  to  30  deg.  higher  than  its 
reversal  Ar3,  while  Ac2  occurs  at  nearly  the  same  temperature  as  Ar2. 

Thermal  Critical  Points  in  Very  Low  Carbon  Steel.  —  Let  us  now  take  a  sample 
of  steel  containing  some  0.10  per  cent  carbon,  and  let  us  ascertain  its  rate  of  cooling 
from  a  high  temperature  precisely  as  before.  Three  thermal  retardations  will  be  de- 
tected, Ar3  at  about  850  deg.,  Ar2  near  750  deg.,  and  Ari  (point  of  recalescence)  near 
675  deg.  Of  these  three  spontaneous  evolutions  of  heat  the  upper  one  at  A.TS  will  be 
the  most  marked,  while  at  Ar2  and  at  Ari  they  will  be  quite  faint,  their  satisfactory 
detection  calling  for  the  use  of  delicate  instruments  and  careful  manipulations.  On 
heating  corresponding  retardations  will  occur,  due  to  spontaneous  absorptions  of 
heat,  the  resulting  critical  points  being  designated  as  Ac3,  Ac2,  and  Aci.  Of  these  Ac3 
and  Aci  will  occur  at  temperatures  some  25  deg.  or  more  higher  than  Ar3  and  An, 
while  Ac2  will  occupy  nearly  the  same  position  as  Ar2  on  the  temperature  scale,  that 
is  about  750  deg. 

Peculiarities  of  the  Point  A2.  —  The  point  A2  is  generally  less  marked  than  the 
points  A3  and  AI.  Unlike  A3  its  position  is  little  affected  by  the  carbon  content,  and 
unlike  A3  and  AI  the  point  on  heating,  Ac2,  occurs  at  nearly  the  same  temperature  as 
the  point  on  cooling,  Ar2.  To  these  peculiarities  must  be  added  another  one,  namely, 
the  fact  that  A2  appears  to  cover  a  wide  range  of  temperature.  While  its  intensity 
decreases  with  fall  of  temperature  its  lower  limit  probably  extends  to  considerably 
below  700  deg.  In  other  words  the  transformation  of  which  A2  is  a  manifestation  is 
not  completed  by  the  time  the  point  AI  is  reached.  Indeed  Osmond  mentions  550 
deg.  C.  as  the  probable  lower  limit  of  the  point  A2.  Some  explanations  of  these  pecu- 
liarities of  the  point  A2  will  soon  be  offered. 

Thermal  Critical  Points  of  Medium  High  Carbon  Steel.  —  The  determination  of 
the  rate  of  cooling  of  a  steel  containing  some  0.45  per  cent  carbon  reveals  the  exis- 
tence of  two  critical  points,  one,  evidently  the  point  of  recalescence,  Ari,  at  the  usual 
temperature  (650  to  700  deg.)  and  one  upper  point  in  the  vicinity  of  725  deg.  Does 
the  presence  in  this  steel  of  only  one  upper  point  mean  that  one  of  the  two  upper 
points  detected  in  carbonless  iron  and  in  very  low  carbon  'steel  has  disappeared, 
because  of  the  presence  of  more  carbon,  or  does  it  mean  that  the  two  upper  points 
have  now  united  into  a  single  one?  The  latter  view  is  generally  assumed  to  be  the 
correct  one  and  this  single  upper  point  of  medium  high  carbon  steel  is  designated 
accordingly  by  Ar3.2.  This  notation  clearly  implies  that  the  two  distinct  evolutions 
of  heat  which  in  carbonless  iron  and  in  very  soft  steel  occur  separately  at  Ar3  and 
Ar2  here  occur  at  one  and  the  same  temperature.  Increasing  the  carbon  content 
decreases  the  interval  of  temperature  between  the  two  upper  points  until,  finally,  for 
a  certain  carbon  content  the  points  meet  to  form  the  double  point  Ar3.2. 

Merging  of  A3  and  A2.  —  It  has  been  seen  that  as  the  carbon  increases  the  point 
A3  is  gradually  lowered  until  finally  it  merges  with  A2,  whose  position  is  not  greatly 
affected  by  the  presence  of  carbon,  to  form  the  point  A3.2.  It  would  be  interesting  to 
know  the  exact  proportion  of  carbon  required  to  cause  this  merging.  This,  however, 
is  difficult  to  ascertain  because  of  the  experimental  difficulty  of  separating  two  crit- 
ical points  situated  very  near  each  other  as  they  must  be  in  the  vicinity  of  the  mer- 
ging point,  and  also  because  this  merging  will  be  shifted  somewhat  by  speed  of  heating 
and  cooling  and  by  slight  changes  of  chemical  composition.  From  the  mass  of  experi- 


LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL  7 

mental  evidences  which  has  been  published  it  seems  probable  that  the  merging  takes 
place  at  about  0.30  per  cent  carbon. 

Thermal  Critical  Point  in  Eutectoid  Steel.  —  Eutectoid  steel,  that  is  steel  con- 
taining some  0.85  per  cent  of  carbon,  exhibits  but  one  critical  point,  the  point  of  re- 
calescence,  very  marked  at  about  675  deg.  C.  on  cooling.  Shall  it  be  inferred  from 
the  occurrence  of  this  single  point  that  in  eutectoid  steel  the  transformations  of  which 
the  upper  points  A3  and  A2  or  the  double  point  A3.2  are  manifestations  do  not  take 
place?  Or  shall  it  be  assumed  that  these  transformations  now  take  place  at  the  same 
temperature  as  the  transformation  corresponding  to  the  critical  point  A[?  In  other 
words  that  increasing  the  amount  of  carbon  has  so  depressed  the  position  of  the  two 
upper  points  as  to  cause  them  to  unite  with  the  lower  point,  "forming  now  a  triple 
point  to  be  designated  as  Ar3.2.i?  This  is  the  view  generally  held.  The  critical  point 
on  heating  is  designated  by  the  notation  Ac3.2.i.  It  will  be  explained  in  another 
lesson  why  the  points  A3,  A2,  or  A3.2  cannot  exist  in  eutectoid  or  hyper-eutectoid  steel, 
when  it  will  also  be  shown  that  the  single  point  of  eutectoid  steel  is  not  in  fact  a 
merging  of  A3,  A2,  and  AI,  but  merely  the  point  Ai,  the  upper  points  having  disap- 
peared. 

Merging  of  A3.2  and  AI.  —  As  the  carbon  content  of  the  steel  increases  still  more 
after  the  merging  of  A3  and  A2  has  been  effected,  the  interval  between  the  points 
A3.2  and  AI  gradually  diminishes  until  these  two  points,  in  turn,  appear  to  merge  to 
form  the  triple  point  A3.2.i-  Theoretically  this  apparent  merging  should  occur  when 
the  steel  is  composed  entirely  of  pearlite,  that  is,  when  it  contains  in  the  vicinity  of 
0.85  per  cent  carbon,  for  reasons  that  will  later  be  made  clear.  As  a  matter  of  fact, 
however,  the  merging  seems  to  take  place  long  before  so  large  a  proportion  of  carbon 
is  present,  for  the  point  A3.2  is  seldom  detected  in  steel  containing  more  than  some 
0.50  or  0.60  per  cent  of  carbon;  this  is  probably  due,  as  already  explained,  to  the 
difficulty  of  separating,  experimentally,  two  critical  points  so  close  to  each  other. 

Thermal  Critical  Points  in  Hyper-Eutectoid  Steel.  —  Carefully  conducted  obser- 
vations reveal  the  existence  of  an  upper  critical  point  in  hyper-eutectoid  steel,  at 
least  in  steel  containing  a  decided  amount  of  free  cementite,  and,  of  course,  of  the 
point  of  recalescence.  It  seems  proper  to  designate  this  upper  point  by  the  symbol 
Acm  (Arcm  on  cooling,  Accm  on  heating)  for  reasons  later  to  be  given,  cm  stand- 
ing for  cementite.  At  least  one  writer,  however,  has  designated  this  point  on  cooling 
by  the  notation  Armc,  me  standing  for  massive  cementite.  Other  writers  have  called 
it  an  A3  point,  a  notation  from  which  one  would  naturally  infer  that  this  upper  point 
of  hyper-eutectoid  steel  is  similar  to  the  upper  point  of  iron  and  of  very  low 
carbon  steel,  which  is  not  the  case. 

Purely  theoretical  considerations  lead  us  to  infer  that  the  position  of  the  point 
Acm  is  lowered  as  the  proportion  of  carbon  decreases,  finally  merging  with  the  point 
A3.2.i  at  the  eutectoid  point.  It  would  follow  from  this  that  the  single  point  of  eu- 
tectoid steel  is  really  a  merging  of  four  points  A3,  A2,  AI,  and  Acm  and  that  it  should 
accordingly  be  designated  by  A3.2.i  cm.  It  is,  however,  the  universal  custom  to  ignore 
this  contribution  of  Acm  to  the  single  point  of  eutectoid  steel  and  to  use  for  the  latter 
the  notation  A3.2.i. 

The  amount  of  heat  evolved  at  Arcm  is  very  slight,  hence  the  difficulty  of  detect- 
ing this  point.  Carpenter  and  Keeling  ascertained  its  existence  in  steels  containing 
respectively  1.31,  1.51,  1.69,  1.85,  and  1.97  per  cent  carbon  at  the  following  corre- 
sponding temperatures:  883,  911,  985,  1030,  and  1042  deg.  C.  With  lower  carbon 


8  LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 

contents,  that  is  nearer  the  eutectoid  composition,  the  heat  evolved  is  so  slight  that 
the  detection  of  Arcm  as  a  separate  point  is  quite  impossible.  In  theoretical  dia- 
grams, however,  the  existence  of  this  point  is  always  indicated  in  all  hyper-eutectoid 
steels  with  a  sharp  merging  with  A3.2.i  at  the  eutectoid  point. 

The  point  Arcm  then  should  occur  in  all  hyper-eutectoid  steels  at  temperatures 
increasing  from  some  700  to  1050  deg.  C.  as  the  carbon  increases  from  0.85  to  2.00 
per  cent. 

Merging  of  A3.2.i  and  Acm.  —  As  already  explained  theoretically  the  merging  of 
the  points  A3.2.i  and  Acm  should  take  place  at  the  eutectoid  composition,  that  is,  for 
steel  containing  in  the  vicinity  of  0.85  per  cent  carbon.  Experimentally,  however, 
the  point  Acm  cannot  be  detected  in  steel  containing  less  than  some  1.20  per  cent 
carbon.  Bearing  in  mind  that  hypo-eutectoid  steels  containing  more  than  0.60  per 
cent  carbon  or  thereabout  have  likewise  but  one  critical  point  so  far  as  experimental 
evidences  are  concerned,  it  will  be  seen  that  for  all  practical  purposes  we  may  con- 
sider all  grades  of  steel  containing  from  0.60  to  1.20  per  cent  carbon  as  having  but 
one  critical  point,  namely,  the  point  of  recalescence,  at  some  675  deg.  C.  on  cooling, 
although  theoretically  eutectoid  steel  only  should  have  but  one  such  point. 

Minor  Critical  Points.  —  Some  experimenters  believe  to  have  discovered  some  critical  points 
other  than  those  so  far  described.  These  points,  which  may  be  referred  to  as  minor  critical  points, 
correspond  to  very  faint  evolutions  or  absorptions  of  heat,  and  produce,  therefore,  but  very  slight 
jogs  in  the  thermal  curves.  Their  existence  is  not  fully  established  and  they  appear  to  have  but 
little  if  any  influence  upon  the  practical  side  of  our  subject.  They  should,  however,  be  mentioned  in 
these  lessons  so  that  the  student  may  at  least  have  some  idea  of  their  nature  and  claims  to  recogni- 
tion. Roberts-Austen  in  1898  detected  a  slight  evolution  of  heat  between  550  and  600  on  cooling  in 
iron  and  hypo-eutectoid  steel,  and  this  point  was  again  detected  by  Carpenter  and  Keeling  in  1904. 
The  latter  observers  named  it  the  Aro  point,  following  in  this  Roberts-Austen. 

Roberts-Austen  detected  another  evolution  of  heat  in  pure  iron  between  450  and  500  deg.  C. 
the  existence  of  which  he  ascribed  to  the  presence  of  hydrogen  resulting  in  a  separation  of  hydroxide 
of  iron  taking  place  at  this  critical  point.  Finally  the  same  observer  described  one  more  slight  evo- 
lution of  heat  in  pure  iron  at  about  270  deg.  C.  which  he  tentatively  ascribed  to  the  formation  of  an 
iron-iron  hydroxide  eutectic. 

Arnold  believes  in  the  existence  of  a  critical  point  between  A3  and  A2,  of  maximum  intensity 
when  the  steel  contains  some  50  per  cent  of  pearlite  (about  0.45  per  cent  carbon)  which  he  thinks  is 
due  to  the  formation  or  segregation  of  pearlite  and  hardenite,  a  constituent  later  to  be  described. 

Data  Showing  the  Position  of  the  Critical  Points.  —  By  far  the  most  comprehen- 
sive set  of  determinations  of  the  critical  points  of  iron  and  steel  was  made  by  Car- 
penter and  Keeling.  Their  results  are  shown  in  the  table  on  the  following  page. 
The  table  includes  the  critical  points  occurring  during  the  solidification  period  of  the 
various  steels  and  irons  investigated.  These  will  be  considered  in  another  lesson. 

Relative  Quantities  of  Heat  Evolved  or  Absorbed  at  the  Critical  Points.  —  The 
various  critical  points  that  have  been  considered  in  the  preceding  pages  do  not  indi- 
cate evolutions  or  absorptions  of  equal  quantities  of  heat;  they  are  not  of  equal  in- 
tensity. The  point  A3  is  very  marked  and  sharply  denned  in  carbonless  iron  but 
decreases  rapidly  in  intensity  as  the  carbon  increases.  The  point  A2  is  relatively 
feeble  and  not  very  sharply  denned  and  as  already  mentioned  shows  a  tendency  to 
cover  a  considerable  range  of  temperature.  Its  intensity  moreover  is  little  affected 
by  the  carbon  content  of  the  steel.  The  point  A3.2,  being  a  merging  of  A3  and  A2,  is 
more  intense  than  A2  but  less  intense  than  A3  in  carbonless  iron  owing  to  the  fact 
that  when  the  merging  takes  place  the  A3  point  has  lost  much  of  its  intensity.  The 


LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 


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10  LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 

point  Ai  is  feeble  in  very  low  carbon  steel  but  its  intensity  increases  rapidly  with  the 
carbon  content,  becoming  so  great  as  to  cause  the  metal  to  glow  or  recalesce  as  pre- 
viously described  and  being  maximum  for  steel  of  eutectoid  composition.  These 
differences  in  the  thermal  values  of  the  critical  points  will  be  explained  in  another 
lesson. 

Graphical  Representation  of  the  Position  and  Magnitude  of  the  Critical  Points.  — 
The  position  of  the  critical  points  corresponding  to  various  percentages  of  carbon  is 
illustrated  graphically  in  Figure  2.  The  diagram  refers  to  the  critical  points  of  cool- 
ing, i.e.  the  Ar  points,  and  it  should  be  borne  in  mind  that  the  corresponding  points 
on  heating,  the  Ac  points,  occur  some  25  to  50  deg.  higher,  with  the  exception  of 
the  point  Ac2  which  seems  to  occupy  nearly  the  same  position  as  the  point  Ar2.  An 
attempt  has  been  made  in  this  diagram  to  indicate  the  relative  intensities  of  the 
various  points  by  shaded  areas  of  proportional  thickness  on  both  sides  of  the  lines 
indicating  their  position.  This  is  based  chiefly  on  theoretical  considerations  and  is 
in  accordance  with  the  generally  accepted  views  regarding  the  causes  of  the  critical 
points  as  explained  in  the  next  lesson.  An  examination  of  the  diagram  shows  (1) 
that  the  point  Ar3  intense  in  carbonless  iron  decreases  gradually  in  intensity  as  the 
carbon  increases,  (2)  that  the  intensity  of  Ar2  is  not  greatly  affected  by  the  carbon 
content,  (3)  that  Ar3.2  fairly  intense  at  first  becomes  rapidly  feebler  and  finally  dis- 
appears just  as  it  meets  Ar!,  (4)  that  Ari  at  first  very  faint  becomes  more  marked 
with  increased  carbon,  being  maximum  for  a  carbon  content  of  some  0.85  per  cent 
(the  eutectoid  point),  (5)  that  the  point  Ar3.2.i  very  intense  at  the  eutectoid  point 
gradually  loses  some  of  its  intensity,  although  always  remaining  pronounced,  and 
(6)  that  the  point  Arcm  very  faint  near  the  eutectoid  composition  increases  in  in- 
tensity with  the  carbon  content. 

These  theoretical  inferences  are  well  supported  by  experimental  evidences  in  the 
case  of  the  magnitude  of  the  points  Ari  and  Ar3.2.i  and  quite  satisfactorily  in  regard 
to  Ar3  and  Arcm.  They  ascribed  to  the  points  Ar2  and  Ar3.2,  however,  a  magnitude 
and  a  sharpness  which  is  not  borne  out  by  experiments  as  later  explained  when  it 
will  also  be  seen  that  some  writers  doubt  the  accuracy  of  the  explanation  generally 
offered  to  account  for  the  point  A2. 

The  diagram,  therefore,  while  undoubtedly  useful,  is  probably  but  approximately 
accurate  and  likely  to  be  modified  with  increased  knowledge  of  the  facts  it  aims  to 
depict. 

Determination  of  the  Thermal  Critical  Points.  —  The  thermal  critical  points  are 
universally  determined  by  means  of  the  Le  Chatelier  thermo-electric  pyrometer. 
Indeed  it  is  the  invention  of  this  invaluable  instrument  that  made  the  detection  of 
the  upper  critical  points  possible.  Had  it  not  been  invented  we  probably  would  still 
be  in  ignorance  of  the  existence  of  the  upper  points,  while  we  would  have  but  little 
knowledge  of  the  exact  position  of  the  point  of  recalescence.  The  necessary  experi- 
mental manipulations  will  be  found  described  in  the  instructions  given  to  carry  on 
the  experiments  appended  to  this  lesson. 

Cooling  and  Heating  Curves.  —  The  determination  of  the  thermal  critical  points 
calls  for  the  construction  of  heating  and  cooling  curves.  In  these  curves  successive 
falls  (or  rises)  of  temperature,  say  of  10  deg.  C.,  6  -  10,  6  -  20,  0  -  30  .  .  .  are  plotted 
as  ordinates,  while  as  abscissae  are  plotted  (a)  the  corresponding  time  intervals  in  sec- 
onds, t,  t',  t",  t'"  .  .  .  elapsed  since  the  beginning  of  the  observation,  or  (6)  the 
actual  intervals  of  time  t'  -  t,  t"  -  t',  t"'  -  t"  .  .  .  required  for  each  noted  fall  of 


LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 


11 


12 


LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 


temperature.    In  other  words  the  coordinates  are  6  and  t  in  the  first  instance,  6  and 

—  in  the  second.     The  curve  obtained  by  the  first  method  is  known  as  a  time- 
d0 

temperature  curve  while  the  second  method  yields  an  inverse  rate  curve.1 

Time-temperature  curves  representing  the  heating  and  cooling  of  pure  iron  are 
shown  in  Figure  3.     While  in  these  curves  the  evolutions  or  absorptions  of  heat  cor- 


1500 
HOC 
1800 
1200 

t 

0 

£1100 

§1000 
MO 

800 
TOO 
600 
600 

s 

Solid!  11 

nation  To 

nt 

\ 

/ 

V 

Iron 

/ 

\ 

V 

1 

/ 

*\ 

// 

\ 

k 

/ 

// 

*»+ 

/ 

V^SW 

Ar™ 

J 

800° 

Ac2 

t 

\ 

N 

^  /3  Iron 

/ 

1 

srtr 

\ 

\ 

<r  Iron 
\ 

/ 

X 

X 

10             90              80              «             W              CO              .70             80              SO            101 

Wumtes 

Fig.  3.  —  Time-temperature  curves.     Heating  and  cooling  of  pure  iron. 

(Goerens.) 

responding  to  the  points  A3  and  A2  can  be  detected,  they  do  not  stand  out  very 
conspicuously,  and  it  may  well  be  feared  that  slight  thermal  retardations  might 
escape  detection  in  curves  of  this  kind  since  they  would  cause  but  very  slight  jogs 
in  the  curve.  These  considerations  led  Osmond  to  adopt  the  inverse  rate  method 
for  the  plotting  of  thermal  curves.  Curves  of  this  type  are  shown  in  Figures  4  and  5. 
The  thermal  points  correspond  to  sharp  peaks  in  the  curves,  the  lengths  of  which 
are  roughly  proportional  to  the  amount  of  heat  evolved  on  cooling  or  absorbed  on 


1  It  is  evident  that  similar  curves  would  result  from  reversing  the  observations,  i.e.  noting  the 
successive  falls  of  temperatures  9,  e' ,  e"  .  .  .  corresponding  to  equal  intervals  of  time,  say  of  15 
seconds,  t  +  15,  t  +  30,  t  +  45  .  .  .  and  plotting  the  former  as  ordinates  and  the  latter  as  abscissa;. 

The  coordinates  in  this  case  would  be  —  and  t. 

dt 


LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 


13 


Fig.  4.  —  Inverse  rate  curves.      Cooling  of  steels  containing  respectively 
0.02,  0.14,  0.45,  and  1.24  per  cent  carbon.      (Osmond.) 


14 


LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 


heating.    This  method  is  quite  universally  applied  unless,  as  later  explained,  a  neutral 
body  is  used  for  the  determination  of  the  critical  points. 

Use  of  Neutral  Bodies.  —  The  method  described  above  for  the  detection  of  the 
thermal  critical  points  is  open  to  the  objection  that  the  rate  of  cooling  or  heating 
of  the  steel  under  observation  is  necessarily  affected  by  any  irregularity  in  the  cooling 
or  heating  of  the  furnace  itself  and  by  other  outside  agencies.  These  disturbing  fac- 
tors introduce  irregularities  in  the  thermal  curves  which  may  render  their  interpreta- 
tion difficult  and  may  indeed  altogether  hide  the  existence  of  critical  points  where 
but  a  very  small  amount  of  heat  is  evolved  or  absorbed.  Again,  on  cooling  for  in- 
stance, when  a  critical  point  is  reached  the  temperature  of  the  metal  is  affected  in 
opposite  directions  (1)  by  the  cooling  furnace  which  has  a  tendency  to  lower  its  tern- 


TIM 
IN  SIC 

NOS 
70 

70 
60 
50 
40 
30 

20 

10 
0 

LU 
CO 

a 
a 

o 
i 

g 

n^ 

! 

V5$ 

y 

i 

^Q- 

I 

fe 

sfc^- 

•^_ 

:rrrr-- 

^"***~- 

^.u_ 

MANG^.^- 

Jllfi 

_  _      m  „ 

'-.*—! 

—  • 

00*1  150°  1100*  1050°  1000*  950°  000°  850°  800*  750*  700*  650*  600°  550*  500°450°46o°3'50° 

TEMPERATURE. 


Fig.  5.  —  Inverse  rate  curves.     Cooling  of  pure  iron,  low  carbon  steel,  high  carbon 
steel,  and  manganese  steel.     (Roberts-Austen.) 


perature  and  (2)  by  the  evolution  of  heat  occurring  at  the  critical  point,  the  effect  of 
which  is  to  raise  its  temperature.  It  is  evident  that  the  cooling  influence  of  the  fur- 
nace has  a  tendency  to  decrease  the  apparent  magnitude  of  the  critical  point  and 
therefore  to  mask  it. 

The  elimination  of  these  objectionable  influences  should  result  in  sharper  thermal 
curves  and  in  the  detection  of  faint  evolutions  or  absorptions  of  heat.  This  was  ac- 
complished by  Roberts-Austen  through  the  use  of  a  neutral  body  and  double  thermo- 
couple so  connected  that  the  difference  of  temperature  between  the  metal  under 
investigation  and  the  neutral  body  is  recorded,  as  well  as  the  actual  temperature  of 
the  metal.1  If  the  heat  capacities  and  emissivities  of  the  metal  and  of  the  neutral 
piece  were  identical  their  temperature  would  be  exactly  the  same  except  at  the  crit- 
ical points,  when  heat  is  evolved  or  absorbed  by  the  metal  while  the  neutral  body  is,  of 

1  For  the  arrangement  of  the  galvanometers,  connections,  etc.,  see  the  description  of  the  Saladin- 
Le  Chatelier-Pellin  instrument  described  under  "Apparatus"  and  the  description  of  other  instru- 
ments using  neutral  bodies  in  an  appendix  to  these  lessons. 


LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 

900      gOO       700       600       500  900      800       700       600      500 


C  =  0,06 

Mn=o,3i 


15 


00,10 


C=O.I5 


0  =  0.22 

Mn=0,53 


C=  0,36 
Mn^O,  52 


\J 


C- 0,4-8 
Mn--0.45 


~=0,  66 
In  =  0.7 1 


900°   V00°    700°   600'     soo° Temperatures      300'    foo'    700'     6OO' 

Fig.  g,  —  Difference  curves.     Cooling  and  heating  of  various  steels.      (Saladin.) 

course,  free  from  any  such  thermal  disturbance.  Any  difference  of  temperature 
between  the  two  pieces,  therefore,  would  indicate  a  critical  point.  Since  it  is  generally 
impossible,  however,  to  use  a  neutral  body  having  exactly  the  same  heat  capacity  as 
that  of  the  metal  under  observation,  it  will  be  apparent  that  there  will  always  be  a 
difference  between  the  temperatures  of  the  two  pieces,  one  always  lagging  behind  the 


16 


LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 


other,  and  that  the  critical  points  will  correspond  to  sudden  increase  in  the  difference 
between  their  respective  temperatures.  Since,  however,  the  critical  points  are  now 
caused  solely  by  abrupt  differences  between  the  temperatures  of  the  two  bodies,  they 
are  freed  from  the  irregularities  mentioned  above  as  well  as  from  the  masking  in- 
fluence of  the  falling  or  rising  temperature  of  the  furnace,  seeing  that  both  pieces  are 
now  equally  affected,  and  the  curves  obtained  should  indicate  more  sharply  and  con- 
spicuously the  existence  of  even  faint  absorptions  or  evolutions  of  heat. 

The  neutral  body  should,  of  course,  be  free  from  any  thermal  transformation 
within  the  range  of  temperature  covered  by  the  experiments.  Platinum,  porcelain, 
clay,  25  per  cent  nickel  steel,  and  (by  the  author)  austenitic  manganese  steel  have 
been  used.  The  plotting  of  the  thermal  curves  when  a  neutral  body  is  used  may  be 
done  in  two  different  ways,  (1)  successive  falls  (or  rises)  0  -  10,  &  -  20,  0  -  30  ...  of 
the  temperature  of  the  metal  as  indicated  by  one  of  the  galvanometers  may  be  plotted 


tooo      900 


600 


too 


Fig.  7.  —  Difference  curves.     Cooling  and  heating  curves  taken  on  same 
photographic  plate.     (Saladin.) 


as  ordinates  against  the  corresponding  differences  in  temperature  0  -  0\,  6'  -  6'\, 
0"  -  B'\  ...  of  the  two  cooling  bodies,  as  indicated  by  the  second  galvanometer, 
the  coordinates  in  this  case  being  6  and  6  -  61,  and  the  curve  known  as  a  "difference" 
curve,  or  (2)  according  to  Rosenhain,  successive  falls  of  temperature  may  be  plotted 
as  ordinates  against  the  corresponding  rate  of  cooling  for  each  degree  of  temperature 

Q  -  Qi    &   -  6  i   &     -  6   i  ag  akscjssa;)  the  coordinates  being  in  this  method  6  and 
6  6'  6" 

d  (0  ~  QV  and  the  curve  known  as  a  "derived  differential"  curve. 


Difference  curves  are  shown  in  Figures  6,  7,  and  8.  The  curves  of  Figures  6  and  7 
were  taken  with  a  Saladin-Le  Chatelier-Pellin  instrument,  Figure  6  shows  the  crit- 
ical points  on  heating  and  cooling  of  a  series  of  carbon  steels  containing  from  0.06  to 
0.66  per  cent  carbon,  the  cooling  and  heating  curves  having  been  taken  on  separate 
photographic  plates.  In  Figure  7  is  shown  the  heating  and  cooling  curves  taken  on 
the  same  plate  of  two  steels  containing  respectively  0.36  and  0.46  per  cent  carbon. 

The  curves  of  Figure  8  are  difference  curves  of  a  series  of  very  pure  carbon  steels 
taken  by  Carpenter  and  Keeling. 


LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 


17 


Fig.  8.  —  Difference  curves.     Cooling  of  a  series  of  very  pure 
carbon  steels.      (Carpenter  and  Keeling.) 

The  purpose  of  Rosenhain's  derived  differential  method  of  plotting  is  to  eliminate 
the  irregularities  from  which  difference  curves  still  suffer  and  which  are  due  chiefly  to 
differences  between  the  heat  capacities  and  emissivities  of  the  sample  and  neutral 
body  resulting  in  differences  in  their  rates  of  cooling  and  heating.  The  resulting 


18 


LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 


curves  cannot,  of  course,  be  autographically  recorded.    They  call  for  the  replotting 
of  the  data  afforded  by  the  difference  (0  vs.  6  -  61)  curves. 

Additional  Illustrations  of  Cooling  Curves.  —  The  different  types  of  cooling  curves 
described  in  the  preceding  pages  are  well  illustrated  in  Figure  9.  These  curves  were 
constructed  from  the  data  given  in  the  following  table,  in  which  each  unit  in  t  repre- 
sents intervals  of  time  of  15  seconds,  0  the  corresponding  temperatures  of  the  sample, 
and  0  -  0\  corresponding  differences  of  temperature  between  the  sample  and  a  neutral 
body  cooling  under  identical  conditions.  The  curve  B,  representing  the  cooling  curve 
of  a  neutral  body  free  from  critical  transformations,  has  been  added  for  comparison. 


BSD 


64O 


B30 


O20 


BIO 


B 


Temperature-time 
curve. 


At: 

40 

Inverse  rale 
curve. 


e-e, 

Difference 
curve. 


. 

Derived  differen- 
tial curve. 


Fig.  9.  —  Different  types  of  cooling  curves.      (Desch.) 


t 

0 

0-0t 

t 

0 

0  0! 

5 

850.0° 

8.5° 

18 

829.0° 

11.0° 

6 

848.0 

8.5 

19 

825.0 

8.2 

7 

844.7 

7.5 

19.5 

823.3 

7.3 

8 

842.0 

7.0 

20 

822.2 

6.7 

9 

839.5 

6.3 

21 

821.7 

7.7 

10 

-  '838.5 

7.0 

22 

821.5 

8.5 

11 

838.2 

8.8 

23 

821.3 

9.8 

12 

838.1 

10.2 

24 

821.1 

10.1 

13 

838.0 

12.0 

24.5 

819.0 

9.5 

14 

837.9 

13.6 

25 

815.0 

6.0 

15 

837.5 

15.5 

26 

813.0 

5.0 

16 

836.0 

14.6 

27 

811.6 

4.7 

17 

833.0 

13.0 

Self-Recording  Pyrometers.  —  With  the  use  of  neutral  bodies  self-recording  in- 
struments are  generally  employed.  The  Saladin-Le  Chatelier-Pellin  autographic 
pyrometer  has  been  described  under  "Apparatus"  and  other  types  of  self-recording 
instruments  will  be  found  described  and  illustrated  in  an  appendix  to  these  lessons. 
The  self-recording  may  be  by  means  of  photographic  plates  or  by  some  other  mechan- 
ical devices.  The  former  method  calls  for  the  use  of  mirror  galvanometers  sending  a 


LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL  19 

beam  of  light  upon  the  photographic  plate  while  in  other  autographic  recorders 
needle  galvanometers  are  used.  The  relative  merits  between  photographic  recorders 
and  other  types  are  summed  up  by  Burgess  as  follows : 

"It  is  evidently  of  great  advantage  to  use  self-recording  apparatus  when  possible, 
and  it  then  becomes  necessary  to  choose  between  the  photographic  type  and  the 
autographic.  The  latter  possesses  the  advantage  that  the  experimenter  may  watch 
any  part  of  the  record,  and  can  therefore  control  the  operation  and  at  any  moment 
vary  the  conditions  affecting  the  experiment;  whereas  with  a  photographic  recording 
apparatus,  as  usually  constructed,  the  observer  does  not  know  whether  or  not  the 
experiment  is  progressing  properly  until  it  is  finished  and  he  has  developed  the  sen- 
sitive plate.  The  manipulation  by  the  photographic  method  4s  usually  also  more 
delicate  and  time  consuming  and  the  adjustment  less  sure,  and  the  record  often  re- 
quires further  graphical  interpretation.  The  autographic  method  is  in  general  not 
adapted  for  interpreting  phenomena  taking  place  within  an  interval  of  a  few  seconds, 
so  that  for  very  rapid  cooling  it  is  necessary  to  employ  the  photographic  method.  It 
is  possible  to  construct  the  photographic  recorder  so  as  to  obtain  a  very  considerable 
range  of  speeds  with  the  same  apparatus,  while  it  is  difficult  and  costly  to  construct 
an  autographic  recorder  having  more  than  two  speeds." 

Historical.  —  A  brief  historical  sketch  of  the  discovery  of  the  critical  points  of 
iron  and  steel  will  not  be  without  interest.  In  1868  Tschernoff  in  studying  the  har- 
dening of  steel  used  the  notation  A  for  the  temperature  at  which  hardening  by  rapid 
cooling  becomes  suddenly  possible  in  high  carbon  steel.  This  was  the  point  A3.2.i. 

In  1869  Gore  noted  that  at  a  dark  red  heat  steel  exhibited  on  cooling  a  spon- 
taneous dilatation  of  short  duration  followed  by  normal  contraction.  Evidently  the 
point  of  recalescence  An  or  Ar3.2.i. 

In  1873  Barrett  repeated  Gore's  experiments  and  discovered,  on  heating,  a  momen- 
tary contraction  at  nearly  the  same  temperature  as  the  dilatation  on  cooling.  This 
was  the  point  Aci  or  ACS.S.I.  He  further  noted  that  this  dilatation  or  contraction  was 
very  feeble  in  iron  (the  AI  point  in  very  low  carbon  steel)  and  very  marked  in  hard 
steel  (the  point  AS.J.I).  Barrett  also  discovered  the  spontaneous  glow  taking  place 
on  cooling  a  wire  and  gave  it  the  name  of  recalescence. 

In  1879  Barrus  showed  that  the  increase  of  hardness  resulting  from  quenching 
was  not  gradual  but  sudden,  thus  pointing  to  the  existence  of  a  thermal  critical  point 
(the  AI  point). 

In  1885  Osmond  published  his  discovery  of  the  upper  critical  points  As  and  A2  in 
iron  and  low  carbon  steel  and  gave  the  first  accurate  determination  of  the  position  of 
the  point  AI. 


Experiments 

Small  samples  of  steel  should  be  obtained  containing  respectively  some  0.10,  0.25, 
0.50,  0.85,  and  1.50  per  cent  of  carbon,  preferably  J/6  in.  round  or  square  and  1  in. 
long.  A  small  hole,  about  %  in.  in  diameter,  should  be  drilled  in  the  end  of  each 
sample  and  extended  to  the  center  or  even  right  through  the  sample.  The  end  of 
the  thermo-couple  of  a  Le  Chatelier  pyrometer  should  be  inserted  in  these  small 
holes  and  firmly  packed  with  loose  asbestos.  The  sample  thus  attached  to  the  thermo- 
couple should  now  bo  introduced  in  a  suitable  furnace,  preferably  an  electric  resis- 
tance furnace  (see  "Apparatus")  and  gradually  heated  to  a  temperature  of  some 


20  LESSON  VII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 

1000  deg.  C.  While  the  metal  is  thus  being  heated  its  temperature  should  be  observed 
and  the  time  intervals  required  for  each  rise  of  temperature  of  say  10  deg.  care- 
fully recorded.  In  a  similar  way  the  rate  of  cooling  should  be  noted  while  the  metal 
cools  from  1000  to  500  deg.  Inverse  rate  curves  should  be  constructed  which  will 
bring  out  the  evolutions  or  absorptions  of  heat  having  taken  place  during  the 
thermal  treatment.  The  accuracy  of  the  method  greatly  depends  upon  the  care 
and  skill  exercised.  The  experiment  calls  for  the  services  of  two  observers  in  order 
that  one  may  watch  the  galvanometer  while  the  other  notes  and  records  the  corre- 
sponding times. 

There  is  no  difficulty  by  this  method  in  detecting  the  points  Ai  of  medium  carbon 
steel  and  A3.2.i  of  high  carbon  steel  because  very  marked  retardations  occur  at  these 
points.  The  detection  of  the  points  A3  and  A2  and  especially  Acm,  where  but  small 
evolutions  of  heat  are  involved,  on  the  contrary  is  not  always  possible  by  this 
method,  their  satisfactory  detection  calling  for  the  use  of  neutral  bodies  and  self- 
recording  instruments.  The  same  samples  may  be  used. 


Examination 

I.    Describe  the  occurrence  of  the  thermal  critical  points  in  a  steel  containing  0.25 

per  cent  carbon. 

II.  Construct  the  inverse  rate  cooling  curve  of  a  piece  of  steel  whose  cooling  through 
successive  ranges  of  10  deg.  C.  has  required  the  time  intervals  indicated  be- 
low: 

TEMPERATURES  TIME 

DEC.  C.  SECONDS 

750—740 10 

40—30 11 

30—20 12 

20—10 13 

710—700 14 

700—690 16 

90—80 20 

80—70 45 

70—60 20 

60—50 17 

50—40 15 

40—30 16 

30—20 18 

20—10 20 

610—600 23 

III.  Describe  the  influence  of  the  rate  of  heating  and  cooling  upon  the  position  of 

the  critical  points. 

IV.  Explain  why  the  points  Ac3  and  Ar3  do  not  correspond  to  the  same  temperature.    < 

V.     Explain  the  use  of  neutral  bodies  in  the  determination  of  critical  points  by  the 
pyrometric  method. 


LESSON  VIII 

THE  THERMAL  CRITICAL  POINTS  OF  STEEL 

THEIR  CAUSES 

The  thermal  critical  points  described  in  the  preceding  lesson  are  evidently  out- 
ward manifestations  of  internal  transformations  taking  place  spontaneously  at  certain 
critical  temperatures.  We  should  now  inquire  into  the  nature  of  these  transforma- 
tions. 

Let  us  remember  that  there  are  but  three  well-known  causes  of  spontaneous  evo- 
lutions of  heat  in  cooling  bodies  and  of  spontaneous  absorptions  on  heating.  These 
are:  (1)  formation  of  chemical  compounds,  a  phenomenon  which  is  almost  always 
accompanied  by  a  spontaneous  evolution  of  heat  (the  heat  of  formation),  and  the 
reverse  phase,  the  dissociation  of  the  compound  with  absorption  of  heat  (the  heat  of 
dissociation),  (2)  changes  of  state,  that  is,  the  passage  of  a  substance  from  the  solid 
to  the  liquid  or  from  the  liquid  to  the  gaseous  state,  or  directly  from  the  solid  to  the 
gaseous  state,  which  changes  are  always  accompanied  by  spontaneous  absorptions  of 
heat,  and  the  opposite  phases  of  the  same  phenomena,  the  passage  of  a  body  from 
the  gaseous  to  the  liquid  or  from  the  liquid  to  the  solid  state,  when  heat  is  evolved 
(the  latent  heat  of  solidification  in  case  of  a  substance  passing  from  the  liquid  to  the 
solid  state,  etc.)  these  evolutions  or  absorptions  of  heat,  as  the  case  may  be,  main- 
taining the  temperature  of  the  substance  constant  while  a  change  of  state  is  in  progress 
as,  for  instance,  during  solidification  or  melting,  (3)  allotropic  or  polymorphic  trans- 
formations which  are  always  accompanied  by  an  evolution  of  heat  when  the  body 
passes  from  one  allotropic  condition  to  another,  and  by  an  absorption  of  heat  when 
it  returns  to  its  first  allotropic  form  or  vice  versa.  The  meaning  of  allotropy  has  been 
explained  in  Lesson  II.  . 

Causes  of  the  Upper  Points  A3  and  A2  in  Carbonless  Iron.  —  It  has  been  seen  that 
at  these  points  spontaneous  evolutions  or  absorptions  of  heat  occur  in  chemically 
pure  iron  which,  since  they  are  not  accompanied  by  any  change  of  state  (the  metal 
being  considerably  below  its  solidification  point)  must,  it  seems,  necessarily  indicate 
the  existence  of  iron  under  three  allotropic  forms.  It  has  been  mentioned  in  Lesson 
II  that  the  allotropic  form  stable  above  A3  is  known  as  y  (gamma)  iron,  that  stable 
between  A3  and  A2  as  ft  (beta)  iron,  and  the  form  stable  below  A2  as  a  (alpha)  iron. 
The  following  then  takes  place  during  the  cooling  and  heating  of  pure  iron:  as  the 
metal  cools  from  a  high  temperature  when  the  point  Ar3  is  reached,  it  passes  from 
the  gamma  to  the  beta  condition  with  evolution  of  heat,  while  at  Ar2  it  passes  from 
the  beta  to  the  alpha  form  also  with  evolution  of  heat.  On  heating  the  reversals 
take  place,  the  iron  passing  with  absorptions  of  heat  from  the  alpha  to  the  beta  con- 
dition at  Ac2  and  from  the  beta  to  the  gamma  condition  at  Acs.  That  the  point  AS 
indicates  an  allotropic  transformation  is  universally  admitted,  no  one  doubting  the 

l 


LESSON   VIII  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


existence  of  iron  in  at  least  two  allotropic  conditions.  Most  authoritative  writers 
believe  with  Osmond  that  the  point  A2  also  indicates  an  allotropic  transformation, 
and  that  iron,  therefore,  assumes  three  distinct  allotropic  forms,  as  explained  above. 


free 


free  Alpha_ 
ferr-ite 


r~err/te 


A/ 


D 


_  /-  /qu/d   Solufron  of 
/fon    crncj  Car~hon 


Solid/ f /'cat ion  poitrt 


-5o//iy  -So/uf/on  o/ 
ofrrj  mcf  /f~<on  and 
Ccrr-ibon  CAc/stsnite) 


Solution 
CA  usfert/te  > 


5o//'cf  So/of /on 


-A, 


P&ar/jfe 


jB  /-/       C     Temperature 

Fig.  1.  —  Diagram  depicting  structural  changes  in  0.20  per  cent  carbon  steel 
as  it  cools  slowly  from  the  molten  condition  to  atmospheric  temperature. 

As  eminent  an  observer  as  Le  Chatelier,  however,  has  expressed  doubts  as  to  the 
allotropic  character  of  the  point  A2.  His  reasons  will  be  considered  later.  In  these 
lessons  iron  will  be  assumed  to  exist  in  three  allotropic  conditions,  of  which  A3  and 


LESSON  VIII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL  3 

A2  are  the  transformation  points,  this  being  the  generally  accepted  theory  and,  in 
the  author's  opinion,  the  most  probable  one. 

Causes  of  the  Upper  Critical  Points  A.,  and  A2  in  Low  Carbon  Steel.  —  As  might 
be  expected  the  points  A3  and  A2  occurring  in  very  low  carbon  steel  also  indicate 
allotropic  changes  in  the  metal.  According  to  most  writers,  however,  only  that  por- 
tion of  the  metal  which  at  ordinary  temperature  exists  as  free  ferrite  is  here  affected. 
To  make  this  matter  clear  it  will  be  necessary  to  anticipate  somewhat  our  subject, 
\vhile  the  diagram  shown  in  Figure  1  will  be  useful. 

In  this  diagram  the  three  critical  points  of  steel  containing  0.20  per  cent  carbon, 
A3,  AS,  and  AI,  as  well  as  its  solidification  point  and  atmospheric  temperature  are 
represented  by  parallel  lines  drawn  at  suitable  intervals  in  the  scale  "of  temperature. 
The  metal  is  represented  by  the  rectangular  area  ABCD.  The  diagram  illustrates 
the  following  facts  later  to  be  discussed  at  greater  length:  (1)  in  the  molten  condi- 
tion steel  is  considered  to  be  a  liquid  solution  of  iron  and  carbon,  (2)  on  reaching  its 
solidification  point  the  metal  is  converted  into  a  solid  solution  of  gamma  iron  and 
carbon  known  as  austenite,  (3)  upon  reaching  Ar3  some  ferrite  begins  to  be  set  free, 
(4)  the  ferrite  as  it  is  set  free  assumes  the  beta  state,  this  liberation  of  ferrite  and  its 
allotropic  transformation  being  probably  one  and  the  same  phenomenon,  (5)  the 
formation  of  free  ferrite  continues  as  the  steel  cools  from  Ar3  to  Ari,  EFG  represent- 
ing the  ferrite  thus  liberated,  (6)  on  reaching  the  point  Ar2  the  ferrite  liberated  be- 
tween Ar3  and  Ar2,  ML  in  the  diagram,  passes  from  the  beta  to  the  alpha  condition, 
(7)  the  ferrite  liberated  between  Ar2  and  Ari,  LNF  in  the  diagram,  assumes  the  alpha 
condition  (according  to  some  writers)  without  passing  by  the  beta  condition,  while  in 
the  opinion  of  others  the  beta  condition  is  assumed  but  is  immediately  followed  by 
the  alpha  state,  (8)  while  ferrite  is  being  set  free,  the  balance  of  the  steel,  EKIF, 
(according  to  most  writers)  preserves  its  condition  of  solid  solution,  gamma  iron 
plus  carbon,  (9)  upon  reaching  the  point  Art  the  residual  solid  solution,  FIi,  is  con- 
verted bodily  into  pearlite,  (10)  from  Art  down  to  atmospheric  temperature  no  fur- 
ther structural  change  takes  place,  the  steel  being  finally  made  up  of  BH  =  GF  per 
cent  ferrite  and  of  HC  =  FI  per  cent  pearlite. 

On  heating  the  opposite  changes  take  place:  (1)  at  Aci  transformation  of  FI 
pearlite  into  FI  solid  solution  (gamma  iron  +  carbon  =  austenite),  while  this  solid 
solution  begins  immediately  to  assimilate  some  of  the  free  ferrite,  which  as  it  is  as- 
similated passes  to  the  gamma  condition,  (2)  between  AI  and  A3  absorption  of  free 
ferrite  continues,  being  completed  at  Acs,  (3)  on  reaching  the  point  Ac2  the  ferrite, 
ML  in  the  diagram,  which  has  not  been  absorbed  between  Aci  and  Ac2  now  passes  to 
the  beta  condition.  This  diagram  depicts  accurately  the  generally  accepted  views  in 
regard  tc  the  meaning  of  the  critical  points.  If  these  views  are  correct  several  inter- 
esting inferences  may  be  drawn  as  to  the  relative  intensities  of  the  critical  points. 
The  point  A3  in  low  carbon  steel  does  not  indicate  a  complete  transformation,  as  too 
often  loosely  stated,  but  merely  the  beginning,  at  Ar3,  or  the  end,  at  Ac3,  of  a  trans- 
formation extending  over  a  considerable  range  of  temperature,  i.e.  from  AI  to  A3. 
Theoretically,  therefore,  it  would  seem  as  if  the  point  A3  must  correspond  to  a  mere 
change  of  direction  in  cooling  and  heating  curves  rather  than  to  well-marked  jogs. 
The  fact  that  a  decided  jog  marks  the  point  Ar3  in  very  low  carbon  steel  might  be 
ascribed  to  hysteresis,  the  metal  cooling  to  a  temperature  below  that  at  which  the 
AS  change  is  due  so  that  when  the  transformation  begins  to  take  place  it  does  so  with 
added  intensity,  hence  the  jog.  The  jog  corresponding  to  the  Ac3  point  of  very  low 


4  LESSON   VIII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 

carbon  steel  is  not  so  readily  explained.  But  is  not  this  jog  much  less  pronounced 
than  the  one  corresponding  to  Ar3?  The  point  Ar«  marks  (1)  a  complete  transforma- 
tion, namely,  the  passage  from  the  beta  to  the  alpha  state  of  the  free  ferrite  liberated 
between  Ar3  and  Ar2  and  (2)  the  beginning  of  a  transformation,  namely,  the  passage 
to  the  alpha  condition  of  the  ferrite  which  continues  to  be  liberated  between  Ar2  and 
Ari.  Because  of  the  complete  transformation  implied  by  the  point  A2  we  readily 
understand  that  it  should  correspond  to  a  jog  both  in  the  heating  and  cooling  curves, 
and  since  Ar2  is  due  chiefly  to  the  allotropic  transformation  of  the  ferrite  liberated 
between  Ar3  and  Ar2,  we  readily  understand  why  it  should  occur  at  nearly  the  same 
temperature  regardless  of  the  carbon  content.  In  the  light  of  what  precedes,  how- 
ever, the  point  A3  in  steel  instead  of  being  regarded  as  the  manifestation  of  trans- 
formations occurring  and  completing  themselves  at  a  certain  temperature,  in  reality 
indicates  the  beginning  or  end  of  transformations  extending  over  considerable  range 
of  temperature,  namely,  from  A3  to  AI. 

Cause  of  the  Point  A3.2.  —  It  has  been  seen  that  the  point  A3.2  is  apparently  a 
merging  of  the  points  A3  and  A2  of  lower  carbon  steel  and  it  seems  natural  to  infer 
that  the  transformations  which  these  points  indicate,  namely,  the  two  allotropic 
changes,  are  here  likewise  merged,  that  is,  that  they  now  take  place  at  the  same  tem- 
perature. In  other  words  that  when  the  point  Ar3.2  is  reached  on  cooling  the  iron 
passes  from  the  gamma  to  the  beta  and  then  immediately  to  the  alpha  state,  the 
heat  evolved  being  due  to  this  double  allotropic  transformation.  Some  writers  have 
claimed,  however,  that  at  the  point  Ar3.2  the  iron  passes  directly  from  the  gamma  to 
the  alpha  condition,  the  change  of  gamma  to  beta  being  suppressed  in  steel  contain- 
ing over  0.35  per  cent  carbon  or  thereabout.  If  such  hypothesis  were  true  it  would 
have  some  important  bearing  upon  the  probable  theory  of  the  hardening  of  steel  as 
explained  in  another  lesson.  In  the  author's  opinion  the  more  generally  accepted 
view  is  better  supported  by  experimental  facts  and  other  evidences  and  in  these 
lessons  the  point  A3.2  will  be  considered  as  implying  a  double  allotropic  change.  Most 
metallographists  believe  that  like  the  independent  points  A3  and  A2  the  double  point 
A3.2  is  the  result  of  allotropic  changes  affecting  the  free  ferrite  only. 

This  setting  free  and  allotropic  transformation  of  ferrite  is  depicted  diagrammati- 
cally  in  Figure  2  in  the  case  of  steel  containing  0.60  per  cent  carbon  and  having,  there- 
fore, the  two  critical  points  A3.2  and  AL  EOF  indicates  the  gradual  liberation  of 
ferrite  and  its  conversion  to  the  alpha  state  as  the  metal  cools  from  Ar3.2  to  Ari,  the 
steel,  after  complete  cooling,  being  made  up  of  BH  =  GF  per  cent  ferrite  and  CH  =  FI 
per  cent  pearlite. 

If,  as  generally  stated,  the  allotropic  transformation  of  which  A3.2  is  a  manifesta- 
tion affects  only  the  free  (pro-eutectoid)  ferrite,  the  intensity  of  the  point  A3.2  must 
decrease  rapidly  with  decreasing  pro-eutectoid  ferrite,  i.e.  as  the  eutectoid  composi- 
tion is  approached,  and  this  point  must  vanish  altogether  as  it  meets  the  point  AI 
(see  Lesson  VII,  Fig.  2),  from  which  it  further  follows  that  the  single  point  of  eutec- 
toid steel  is  not  in  reality  a  triple  point  as  the  notation  A3.2.i  would  imply,  resulting 
from  the  merging  of  AI  and  A3.2,  but  that  on  the  contrary  it  remains  a  single  point, 
being  merely  the  continuation  of  the  AI  point  of  hypo-eutectoid  steel. 

Cause  of  the  Point  AI.  —  It  has  been  seen  that  the  point  AI  does  not  occur  in 
carbonless  iron,  only  feebly  in  iron  containing  little  carbon,  and  with  increased  in- 
tensity as  the  carbon  increases  to  the  eutectoid  point.  The  conclusion  seems  irre- 
sistible that  the  point  AI  must  be  closely  related  to  the  carbon,  that  it  must  indicate 


LESSON  VIII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL  5 

a  sudden  change  in  its  condition.  If  steel  be  rapidly  cooled  from  above  the  point  Aci 
and  then  treated  with  certain  dilute  acids,  practically  all  the  carbon  escapes  as  hy- 
drocarbons, whereas  the  same  steel  after  slow  cooling  through  Ari  when  similarly 


•5o//c/    solution 
(Austen/ fe) 


Tempera-furs 


Fig.  2.  —  Diagram  depicting  structural  changes  in  0.60  per  cent  carbon  steel 
as  it  cools  slowly  from  the  molten  condition  to  atmospheric  temperature. 

treated  yields  a  carbonaceous  residue,  which  upon  being  analyzed  is  found  to  consist 
of  the  carbide  Fe3C.  It  is  assumed  that  upon  quick  cooling  we  retain  the  carbon, 
partially  at  least,  in  the  form  in  which  it  normally  exists  above  AI,  and  seeing  that 


6  LESSON   VIII  —  THE   THERMAL   CRITICAL   POINTS   OF   STEEL 

when  subjected  to  a  similar  treatment  this  carbon  behaves  so  very  differently  from 
the  carbon  of  slowly  cooled  steel,  the  conclusion  is  very  logical  that  carbon  exists 
above  AI  in  a  different  condition  from  that  normal  below  AI.  Above  A:  it  is  called 
"hardening"  carbon,  below  AI  "cement"  carbon.  On  heating  steel  past  the  point 
A.CI  the  carbon  changes  from  the  cement  to  the  hardening  condition,  and  vice  versa 
on  cooling  at  Ari  from  the  hardening  to  the  cement  condition.  It  is,  moreover,  gen- 
erally believed  that  this  hardening  carbon  is  carbon  in  solid  solution  in  the  iron.  If 
it  be  so  the  heat  evolved  at  Ari  is  clearly  in  part  at  least  the  heat  of  formation  of 
the  carbide  Fe3C  and  the  heat  absorbed  at  Aci  clearly  the  heat  of  dissociation  of  that 
carbide  which  now  is  resolved  again  into  its  elements  according  to  the  reversible 

rcaction 


The  intensity  of  the  AI  point  should  then  increase  as  the  carbon  increases  or, 
rather,  as  the  amount  of  pearlite  increases,  and  should  be  maximum,  therefore,  at  the 
eutectoid  point  as  indicated  in  Figure  2,  Lesson  VII.  With  higher  carbon  content  it 
diminishes  slightly  because  the  free  (pro-eutectoid)  cementite  which  is  now  present 
takes  no  part  in  the  transformation  occurring  at  Ari,  having  been  formed  at^a  higher 
temperature,  namely,  at  Arcm,  as  later  explained. 

It  would  seem  that  the  cause  of  the  AI  point,  i.e.  the  point  of  recalescence,  is  in 
this  way  explained  in  a  perfectly  satisfactory  manner.  The  correctness  of  this  theory 
appears  to  be  further  supported  by  microscopical  evidences  which  reveal  the  presence 
of  Fe3C  in  slowly  cooled  steel  while  pointing  to  the  probable  absence  of  it  in  suddenly 
cooled  steel. 

In  recent  years,  however,  it  has  seemed  more  and  more  probable  to  students  of 
metallography  that  it  is  not  carbon  in  its  elementary  state  which  is  dissolved  in 
iron  at  a  high  temperature,  but  rather  the  carbide  Fe3C  itself  and  that  the  difference 
between  the  behavior  of  the  carbon  in  hardened  steel  and  in  slowly  cooled  steel  might 
well  be  satisfactorily  accounted  for  on  the  ground  that  in  hardened  steel  Fe3C  is  dis- 
solved in  iron  and  in  that  form  is  much  more  readily  acted  upon  by  acids,  being  there- 
by converted  into  hydrocarbons,  whereas  Fe3C  when  in  the  free  crystallized  condition. 
as  in  slowly  cooled  steel,  resists  the  action  of  the  acids  and  remains  undissolved.  If  it 
is  Fe3C  and  not  C  which  is  dissolved  in  iron  above  the  critical  range,  it  is  evident 
that  the  point  Ari  cannot  be  caused  by  the  formation  of  Fe3C.  But  it  may  well  be 
due  to  the  crystallization  or  falling  out  of  solution  of  Fe3C.  To  be  sure  this  is  a  fall- 
ing out  of  a  solid  solution,  but  cannot  we  conceive  that  the  falling  out  of  a  constit- 
uent of  a  solid  solution  is  accompanied  by  an  evolution  of  heat  even  if  it  does  not 
imply  a  change  of  state?  In  other  words  is  it  not  possible,  or  even  probable,  that 
crystallization  in  the  solid  state  is  accompanied  by  an  evolution  of  heat?  Surely  this 
crystallization  implies  an  allotropic  or  at  least  a  polymorphic  transformation  and 
are  not  such  transformations  always  accompanied  by  heat  evolutions? 

The  author  offers  these  thoughts  as  possibly  worthy  of  attention  and  as  a  possible 
explanation  of  the  evolution  of  heat  at  Art  if  we  assume  that  Fe3C  and  not  C,  as  it 
now  seems  so  probable,  is  dissolved  in  iron  above  that  point. 

The  Point  A!  an  Allotropic  Point.  —  Most  writers  describe  the  point  AI  as  purely 
a  carbon  point,  that  is,  a  manifestation  of  a  change  affecting  the  condition  of  the 
carbon  only  as  explained  in  the  foregoing  pages.  These  same  writers,  however,  as- 
sert that  the  upper  critical  points,  A3  and  A2  in  low  carbon  steel  or  A3.2  in  higher  car- 
bon steel,  affect  only  the  condition  of  the  free  (pro-eutectoid)  ferrite.  In  this  they 


LESSON   VIII  — THE   THERMAL   CRITICAL   POINTS  OF   STEEL  7 

are  inconsistent,  for  if  the  upper  point  or  points  indicate  allotropic  transformation  of 
the  free  ferrite  only  then  the  lower  point  At  is  decidedly  an  allotropic  point  seeing 
that  it  corresponds  to  allotropic  transformations  of  the  pearlite-ferrite  and  that  in 
steel  containing  more  than  some  0.40  per  cent  carbon  there  is  more  pearlite-ferrite 
than  free  ferrite.  In  other  words  the  point  AI  is  always  an  allotropic  point  indicating 
an  allotropic  transformation  of  the  pearlite-ferrite  similar  to  the  allotropic  transfor- 
mation of  the  free  ferrite  occurring  at  the  upper  points,  and  in  case  of  steel  with 
more  than  0.40  per  cent  carbon  the  allotropic  change  taking  place  at  AI  affects  a  larger 
bulk  of  iron  than  the  change  at  A3.2.  To  make  the  matter  clear  let  us  consider  (Fig.  2) 
a  steel  containing  some  0.60  per  cent  of  carbon  and,  therefore^made  up  after  slow 
cooling  of  72  per  cent  of  pearlite  and  28  per  cent  of  free  ferrite.  This  steel  will  con- 
tain about  72  x  |  =  63  per  cent  of  pearlite-ferrite  represented  by  FO  in  Figure  2. 
When  the  point  AI  is  reached  this  63  per  cent  of  iron  is  still  in  the  gamma  condi- 
tion (according  to  the  general  belief)  and  now  passes  to  the  alpha  condition  either 
directly  or  first  assuming  the  beta  state.  The  allotropic  character  of  the  point  AI  is 
therefore  evident.  Indeed  it  is  sufficient  to  account  for  the  heat  evolved  at  Ari  or 
absorbed  at  Aci  without  the  assistance  of  any  change  occurring  in  the  carbon  condi- 
tion, for  it  is  in  perfect  agreement  with  the  increased  intensity  of  the  point  AI  as  the 
carbon  increases  and  with  its  maximum  at  the  eutectoid  composition,  since  as  the 
carbon  increases  the  amount  of  pearlite  and  therefore  of  pearlite-ferrite  likewise  in- 
creases. 

Summing  up,  three  reasons  may  be  given  for  the  evolution  of  heat  at  Ari:  (1)  for- 
mation of  the  carbide  Fe3C  based  on  the  assumption  that  carbon  as  such  is  dissolved 
in  iron,  (2)  crystallization  of  the  carbide  FesC  based  on  the  assumption  that  this  car- 
bide is  dissolved  in  iron  and  that  crystallization  not  implying  a  change  of  state  may 
produce  heat,  and  (3)  allotropic  transformation  of  the  iron  present  in  austenite  of 
eutectoid  composition.  It  seems  probable  that  both  (2)  and  (3)  contribute  to  the 
heat  developed  at  Ar^ 

Pearlite  Formation.  —  Whatever  differences  of  opinion  may  exist  as  to  the  exact 
cause  or  causes  of  the  evolution  of  heat  corresponding  to  the  point  Ari  all  agree  that 
it  is  due  to  the  transformation  of  austenite  of  eutectoid  composition  (sometimes 
called  hardenite)  into  pearlite,  i.e.  the  conversion  of  a  solid  solution  into  an  aggre- 
gate (ferrite  plus  cementite).  It  is  well  to  bear  in  mind  the  changes  in  the  condition 
of  the  iron  and  carbon  which  this  transformation  seems  to  imply:  (1)  passage  of  the 
iron  from  the  gamma  to  the  beta  condition,  (2)  immediately  followed  by  its  conver- 
sion into  alpha  iron  or,  according  to  some  writers,  (1  and  2)  the  conversion  of 
gamma  iron  directly  into  alpha  iron,  skipping  the  beta  state,  (3)  the  crystallizing  of 
alpha  iron  into  parallel  plates  or  lamellae,  and  (4a)  the  formation  and  crystallizing 
or  (46)  the  crystallizing  only  of  Fe3C  into  parallel  plates  alternating  with  the  ferrite 
plates. 

Cause  of  the  Point  Acm.  —  The  point  Arcm  undoubtedly  indicates  the  beginning 
of  the  liberation  of  free  cementite  in  hyper-eutectoid  steel  as  it  cools  from  Arcm  to 
Ar3.2.i.  This  gradual  formation  of  free  cementite  is  well  shown  in  Figure  3  where  it 
is  represented  by  the  triangle  EFG.  When  the  point  A3.2.t  is  reached  the  residual 
austenite,  now  of  eutectoid  composition,  is  converted  bodily  into  pearlite,  the  steel 
consisting  finally  of  BH  =  GF  per  cent  free  cementite  and  DC  =  FI  per  cent  pearl- 
ite. It  will  be  seen  that  this  upper  point  of  hyper-eutectoid  steel,  like  the  points  A3 
and  A2  of  hypo-eutectoid  steel,  does  not  indicate  a  complete  transformation  but 


8 


LESSON   VIII  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


merely  the  beginning  of  a  transformation  covering  a  wide  range  of  temperature, 
namely,  from  Acm  to  AS.Z.I.  If  it  corresponds  to  a  jog  rather  than  to  a  mere  change 
of  direction  in  cooling  curves  this  must  be  ascribed  to  hysteresis  and  its  tendency  to 


D 


L/qt//c/    so/uf/on 
iron  and  carbon 


jotid'ficcrfron 


point 


So//c/ 

gamm  iron  anc/ 

ca/-bon  fAustenifel 


cm 


So//c/  so/ut/on 
(A  ustenife) 


3-L-l 


Fig.  3.  —  Diagram  depicting  structural  changes  in  1.25  per  cent  carbon  steel 
as  it  cools  slowly  from  the  molten  condition  to  atmospheric  temperature. 

accentuate  the  beginning  of  a  transformation  as  previously  explained.  The  evolu- 
tion of  heat  corresponding  to  the  liberation  of  free  cementite  may  be  explained  in 
two  ways:  (1)  actual  formation  of  Fe3C,  carbon  and  not  the  carbide  being  in  solution 


LESSON   VIII  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL  9 

above  Arcm,  or  (2)  crystallization  or  falling  out  of  solution  of  Fe3C  based  on  the  more 
probable  assumption  that  Fe3C  and  not  C  is  dissolved  by  the  iron  above  Arcm,  and 
on  the  further  assumption  that  this  crystallization  in  the  solid  state,  since  it  evidently 
implies  an  allotropic  or,  at  least,  polymorphic  change,  must  be  accompanied  by  an 
evolution  of  heat. 

Since  in  hyper-eutectoid  steel  containing  even  as  much  as  1.5  per  cent  carbon 
there  is  but  a  small  proportion  of  free  cementite  (some  11  per  cent)  the  point  Arcm  is 
caused  by  the  evolution  of  but  a  small  amount  of  heat  and  must  therefore  be  faint. 
Its  intensity,  moreover,  must  decrease  as  the  carbon  decreases  and  the  point  must 
disappear  altogether  as  the  eutectoid  composition  is  reached,  that  is,  just  as  it  meets 
the  single  point  of  eutectoid  steel.  This  is  indicated  in  Figure  2  of  Lesson  VII. 

Allotropy  of  Cementite.  —  If  we  believe,  as  most  metallographists  now  do,  that 
FesC  and  not  C  forms  a  solid  solution  with  carbon  above  the  point  AI  or  A3.2.i,  it 
follows  that  this  dissolved  Fe3C  crystallizes  or  falls  out  of  solution  at  certain  critical 
temperatures,  namely,  Arcm  for  the  free  cementite  of  hyper-eutectoid  steel  and  Ari 
(or  Ar3.2.i)  for  the  pearlite-cementite  of  all  steels,  and  that  this  crystallization  is  ac- 
companied by  an  evolution  of  heat.  This  falling  out  of  solution  really  implies  a 
spontaneous  change  of  crystalline  form  and  is  therefore  an  evidence  of  polymorphism, 
hence  of  allotropy,  for  if  allotropy  does  not  necessarily  imply  polymorphism,  poly- 
morphism implies  allotropy.  Are  we  not  then  justified  in  believing  that  Fe3C  may 
exist  under  two  allotropic  forms:  (1)  an  allotropic  variety  soluble  in  iron,  which  we 
may  call  gamma  cementite  and  (2)  an  allotropic  variety  insoluble  in  iron,  which  we 
may  call  alpha  cementite,  constituting  the  free  cementite  of  hyper-eutectoid  steel 
and  the  hard  plates  of  pearlite? 

The  fact  that  the  crystallizing  or  falling  out  of  solution  of  free  ferrite  in  hypo- 
eutectoid  steel  implies  an  allotropic  transformation  of  the  liberated  ferrite,  points 
with  force  to  a  similar  transformation  forming  part  of  the  liberation  of  free  cementite 
in  hyper-eutectoid  steel.  In  the  case  of  iron  we  are  able  to  actually  prove  this  allo- 
tropy through  the  cooling  of  very  pure  iron  and  the  testing  of  its  properties  while  in 
the  case  of  cementite  such  direct  proof  is  not  yet  at  hand  because  of  the  difficulty  of 
obtaining  pure  cementite  and  of  testing  it  after  producing  it. 

To  generalize,  it  would  seem  as  if  the  crystallizing  or  falling  out  of  solution  of  a 
substance  at  certain  critical  temperatures  always  implies  a  spontaneous  change  of 
crystalline  form  and,  therefore,  an  allotropic  transformation  of  the  substance  separa- 
ting from  the  solution,  whether  that  solution  be  liquid  or  solid.  In  the  former  case 
the  falling  out  of  solution  implies  a  change  of  state,  the  separating  substance  passing 
from  the  liquid  to  the  solid  state,  but  does  it  make  it  less  of  an  allotropic  change? 
Allotropic  transformations  which  also  imply  changes  of  state  might  be  called  in- 
stances of  major  allotropy  to  distinguish  them  from  those  instances  in  which  changes 
of  internal  energy  are  not  accompanied  by  changes  of  state. 

Cause  of  the  Point  A3.2.i  in  Eutectoid  Steel.  —  In  the  case  of  eutectoid  steel  the 
solid  solution  (austenite)  is  originally  of  eutectoid  composition  and,  therefore,  on 
cooling  reaches  the  point  Ar3.2.i  without  rejecting  either  ferrite  or  cementite,  hence 
the  absence  of  upper  points  in  eutectoid  steel.  On  passing  through  the  point  Ar3.2.i 
(Fig.  4)  this  austenite  is  converted  into  pearlite.  Pearlite  contains  87.50  per  cent  of 
ferrite  which  undergoes  allotropic  transformation  at  Ar3.s.i,  hence  the  intense  allo- 
tropic character  of  this  point.  The  crystallizing  of  cementite  probably  contributes 
also  to  the  heat  evolved  at  Ar3.2.i  whether  it  implies  the  formation  of  Fe3C  or 


10 


LESSON   VIII  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


not  as  explained  before.     It  has  also  been  argued  that  this  crystallizing  of  cementite 
probably  implies  an  allotropic  transformation,  in  which  case  the  point  A3.2.i  (and  AI 


3-i-l 


_ 
iron   and 


so/idtf/cof/on 


/OO//7/ 


5o//cf 
-gamma  iron  and 
carbon  (Ausfen/fe) 


-Pear/if  e 


A  //T?  o.s/1  herjc 


£       •  C 

Fig.  4.  —  Diagram  depicting  structural  changes  in  eutectoid  steel  as  it  cools 
slowly  from  the  molten  condition  to  atmospheric  temperature. 

as  well)  would  be  solely  an  allotropic  point,  resulting  from  the  simultaneous  allo- 
tropic transformation  of  both  the  iron  and  the  FesC  of  austenite  of  eutectoid 
composition. 

Finally  let  us  bear  in  mind  that  notwithstanding  its  notation  the  single  point  of 


LESSON   VIII  — THE   THERMAL   CRITICAL   POINTS   OF  STEEL  11 

eutectoid  steel  does  not  in  fact  result  from  the  merging  of  the  several  points  of 
hypo-eutectoid  steel,  being  merely  a  continuation  of  the  point  AI.  This  point  Ai 
is  essentially  a  "pearlite"  point  while  the  upper  points  of  hypo-eutectoid  steel 
are  "ferrite"  points  and  the  upper  point  of  hyper-eutectoid  steel  is  a  "cementite" 
point. 

Cause  of  the  Point  A3.2.i  in  Hyper-Eutectoid  Steel.  —  The  point  A3.2.i  in  hyper- 
eutectoid  steel  is  of  exactly  the  same  nature  as  the  point  A3«.i  of  eutectoid  steel  and 
the  point  A!  of  hypo-eutectoid  steel.  It  marks  the  formation  of  pearlite,  bearing  in 
mind  the  various  changes  in  the  condition  of  the  carbon  and  iron  implied  by  that 
formation.  As  the  proportion  of  pearlite  now  decreases  with  "increase  of  carbon  the 
intensity  of  the  point  A3.2.i  likewise  diminishes. 

Formation  of  Beta  Iron.  —  Allusion  has  been  made  in  these  pages  on  several 
occasions  to  the  different  opinions  entertained  as  to  the  formation  of  beta  iron  in  steel 
of  various  carbon  contents.  The  matter  should  receive  additional  attention.  The 
following  views  are  held:  (1)  iron  in  carbonless  iron  and  in  all  grades  of  steel  passes 
from  the  gamma  to  the  beta  condition  before  assuming  the  alpha  state,  the  absence 
of  an  independent  A2  point  in  medium  high  and  in  high  carbon  steel  notwithstand- 
ing, (2)  iron  passes  from  the  gamma  to  the  beta  condition  only  in  those  grades  of  iron 
and  steel  which  exhibit  the  point  A2,  and,  therefore,  only  in  carbonless  iron  and  in 
steel  containing  less  than  some  0.30  per  cent  carbon,  in  higher  carbon  steel  the  iron 
passing  directly  from  the  gamma  to  the  alpha  state,  (3)  the  beta  condition  does  not 
exist  even  in  carbonless  iron,  the  point  A2  not  being  an  allotropic  point. 

Of  these  three  different  views  the  first  is  the  one  most  generally  accepted  and,  in 
the  author's  opinion,  best  supported  by  evidence.  It  will  be  seen  in  another  lesson 
to  afford  the  most  acceptable  theory  of  the  hardening  of  steel.  The  second  view  is 
based  entirely  upon  the  absence  of  the  A2  point  in  steel  containing  more  than  0.30 
per  cent  carbon,  a  very  weak  foundation,  for  there  is  no  reason  why  the  points  A3.2 
and  A3.2.i  cannot  include  the  gamma-to-beta  transformation.  The  third  view  ad- 
vanced by  Le  Chatelier  will  require  more  convincing  arguments  in  order  to  uproot 
the  belief  that  the  point  A2,  occurring  as  it  does  in  the  purest  iron  and,  as  will  be 
shown  later,  marking  a  sudden  and  pronounced  change  in  its  magnetic  properties,  is 
an  allotropic  point. 

Summary.  —  The  apparent  causes  of  the  thermal  critical  points  of  iron  and  steel 
may  be  briefly  summed  up  as  follows: 

The  point  Ar3  of  carbonless  iron  and  of  steel  containing  less  than  some  0.35  per 
cent  carbon  marks  the  beginning  of  the  liberation  of  ferrite  (which  liberation  con- 
tinues down  to  the  point  Ari)  and  the  transformation  of  that  ferrite  from  the  gamma 
to  the  beta  condition,  this  setting  free  of  ferrite  and  its  allotropic  transformation 
being  probably  simultaneous. 

The  point  Ar2  of  carbonless  iron  and  of  steel  containing  less  than  some  0.35  per 
cent  carbon  indicates  the  transformation  from  the  beta  to  the  alpha  condition  of  the 
ferrite  liberated  between  Ar3  and  Ar2  and  the  beginning  of  the  passage  of  the  ferrite, 
which  continues  to  be  liberated  as  the  metal  comes  from  Ar2  to  Arj,  from  the  gamma 
to  the  beta  and  then  to  the  alpha  condition  or,  as  some  writers  claim,  directly  from 
the  gamma  to  the  alpha  state. 

The  point  Ar3.2  of  steel  containing  somewhere  between  0.35  and  0.85  per  cent 
carbon  marks  the  beginning  of  the  liberation  of  ferrite  which  takes  place  between 
Ar3.2  and  Ar!  and  the  passage  of  that  ferrite  from  the  gamma  to  the  beta  condition 


12 


LESSON   VIII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 


and  immediately  to  the  alpha  state  or,  according  to  some  writers,  directly  from  the 
gamma  to  the  alpha  condition. 

Since  only  the  formation  and  transformation  of  free  ferrite  is  involved  at  the  points 
A3,  A2,  and  As.2  these  points  may  properly  be  called  "ferrite"  points. 

The  point  Ai  of  hypo-eutectoid  steel,  as  well  as  the  point  A3.2.i  of  eutectoid  and 
hyper-eutectoid  steel,  marks  the  rather  sudden  transformation  of  the  residual  solid 
solution  (austenite),  now  of  eutectoid  composition,  into  pearlite,  bearing  in  mind  the 
changes  in  the  conditions  of  the  iron  and  carbon  which  such  transformation  implies. 
The  points  Ai  and  A3.2.i  may  properly  be  called  "pearlite"  points. 

The  point  Arcm  of  hyper-eutectoid  steel  marks  the  beginning  of  the  setting  free 
of  cementite  as  the  metal  cools  from  Arcm  to  Ar3.2.i,  the  liberation  of  cementite 
probably  involving  an  allotropic  change  of  that  constituent  as  previously  explained. 


//OO' 


9 OO° 

Cr/f/ccf/ 
Range 

700' 


f~err/fe 


Solid     So/.uf/on 
/ron    and    Carbon 
(A  usfen/fe) 


c 
f^errtfe 

-/-Austen/fa 


D 


7oC  O       .25 


Pear//fe 


.5 


.35  /O 


Cemenf/fe 


20 


Fig.  5.  —  Diagram  showing  the  relation  between  the  critical  points  and  the  structural 
composition  of  slowly  cooled  steel. 

In  Figure  5  an  attempt  has  been  made  at  showing  diagrammatically  the  relation 
between  the  critical  points  of  steel  and  its  structural  composition  after  slow  cooling. 
It  will  be  readily  understood.  The  upper  part  of  the  diagram  shows  the  location  of 
the  critical  points,  the  lower  part  the  structural  composition  in  percentages  of  ferrite, 
pearlite,  and  cementite.  Taking,  for  instance,  a  steel  containing  0.25  per  cent  carbon  : 
above  A3  at  A  it  is  a  solid  solution  of  carbon  and  iron  (austenite) ;  on  cooling  through 
Ar3  at  B  some  beta  ferrite  is  set  free,  this  liberation  continuing  from  B  to  C;  on  cool- 
ing through  Ar2  at  C  the  free  beta  ferrite  is  converted  into  alpha  ferrite  while  addi- 
tional alpha  ferrite  forms  as  the  metal  cools  to  An,  that  is  from  C  to  D.  The  ferrite 
liberated  on  cooling  from  B  to  D,  that  is  on  cooling  through  the  critical  range,  is  repre- 
sented in  the  lower  part  of  the  diagram  by  DE  which  is  also  the  final  percentage  of 
free  ferrite  in  the  steel.  As  the  metal  cools  through  its  An  point  at  D,  the  residual 


LESSON   VIII  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


13 


austenite,  at  present  of  eutectoid  composition,  is  converted  into  pearlite,  EF  repre- 
senting the  pearlite  here  formed,  i.e.  the  percentage  of  pearlite  in  the  steel.     The 


A 


A.- 


f~ree-     Beta 
ferrtte 


A,- 


A/phct  _ 


A. 


G 


*7-<?e    A/pno_ 
ferr/fe 


D 


_  •So/u^'on  of 

/ron  and  Cor 


of 

•  Go/Tima  /ron  one/ 
Corfoon  {Aust&ntte 


So//c/   5o/uf/'on  of 
~  3efo    /ron   one/  $. 

Car  Aon  (Mortensife  ') 


So//c/    v5o/£///o/?  of 
-  A  Ipho  /r~on  and 
Carbon  f' 


•A. 


A  t 


O  /      /  x—  lerna&ro  f  urc 

D  ri     C 

Fig.  6.  —  Diagram  depicting  structural  changes  in  0.20  per  cent  carbon  steel  slowly 
cooled,  assuming  that  the  iron  remaining  in  solution  as  well  as  the  free  iron  (ferrite) 
undergoes  allo tropic  changes.  To  be  compared  with  Figure  1. 


structural  formation  of  any  steel  can  be  followed  in  the  same  way  in  this  diagram. 
It  will  be  obvious  that  the  vertical  distances  representing  the  percentages  of  ferrite, 
pearlite,  or  cementite  in  any  steel  may  also  be  regarded  as  proportional  to  the  intensities 


14  LESSON   VIII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 

of  the  corresponding  critical  points.  For  instance,  the  distance  ED  may  be  assumed 
to  be  proportional  to  the  combined  intensities  of  the  points  Ar3  and  Ar2  of  a  0.25  per 
cent  carbon  steel  and  the  distance  EF  proportional  to  the  intensity  of  the  AI  point. 
Interpreted  in  this  way  the  diagram  indicates  what  has  already  been  pointed  out:  (1) 
that  the  intensities  of  A3  and  Aa  decrease  as  the  carbon  increases,  these  points  vanishing 
on'  reaching  the  eutectoid  composition,  (2)  that  the  point  AI,  very  faint  at  first,  in- 
creases rapidly  with  increased  carbon,  becoming  maximum  at  the  eutectoid  point  and 
then  decreasing,  and  (3)  that  the  point  Acm,  always  faint,  increases  slightly  as  the 
carbon  increases  above  0.85  per  cent. 

Another  View  of  the  Allotropic  Changes.  —  It  will  be  obvious  from  the  description  of  the 
underlying  causes  of  the  critical  points  given  in  the  foregoing  pages  that,  according  to  the  general 
belief,  iron  must  first  be  freed  from  solid  solution  before  it  can  undergo  any  allotropic  changes  or,  at 
least,  its  liberation  from  solution  and  its  allotropic  transformation  take  place  simultaneously,  the 
latter  never  preceding  the  former.  In  1906  the  author  expressed  the  opinion  that  it  was  far  from 
certain  that  its  liberation  from  solution  must  precede,  or  at  least  be  simultaneous  with,  the  allotropic 
changes  affecting  iron  at  certain  critical  temperatures.  He  ventured  to  put  forward,  in  a  tentative 
way,  the  hypothesis  that  iron  in  solution  might  first  undergo  an  allotropic  transformation  and  then 
be  expelled  in  its  new  allotropic  form.  This  view  was  not  favorably  received  but,  as  it  has  not  been 
shown  to  be  by  any  means  untenable,  the  author  still  believes  it  worthy  of  record  in  these  pages.  It 
is  evident  that  if  the  allotropic  transformation  of  iron  from  the  gamma  to  the  beta  and  then  to  the 
alpha  state  precedes  its  liberation  from  solution,  three  solid  solutions  of  carbon  in  iron  are  formed 
during  the  slow  cooling  of  steel,  namely,  carbon  (or  the  carbide  Fe3C)  dissolved  (1)  in  gamma  iron, 
(2)  in  beta  iron,  and  (3)  in  alpha  iron.  The  first  solid  solution  is  universally  called  austenite  while 
the  hypothesis  leads  almost  irresistibly  to  regarding  the  solid  solution  in  beta  iron  as  martensite  and 
the  solid  solution  in  alpha  iron  as  troostite,  two  constituents  to  be  described  in  another  lesson. 

With  the  assistance  of  the  diagram,  Figure  6,  and  by  comparing  it  with  Figure  1  the  working  of  the 
present  hypothesis  will  be  readily  understood.  In  Figure  6  are  depicted  the  structural  changes  taking 
place  during  the  slow  cooling  of  steel  containing  0.20  per  cent  carbon  and  therefore  exhibiting  the 
three  critical  points  As,  A2,  and  AI.  On  cooling  through  the  point  A3  the  solid  solution  of  carbon 
and  gamma  iron  (austenite)  existing  above  As  is  converted  into  a  solid  solution  of  carbon  in  beta 
iron  (martensite?).  In  the  beta  condition,  however,  iron  cannot  be  retained  in  solution  and  begins 
immediately  to  be  liberated,  and  its  liberation  continues  as  the  metal  cools  down  to  A-,.  Between 
As  and  A2  we  have  a  sr>lid  solution  of  beta  iron  decreasing  in  amount  and  increasing  free  beta  ferrite. 
On  cooling  through  Ar.  both  the  free  beta  ferrite  and  the  dissolved  beta  ferrite  pass  to  the  alpha  si  ,-i  te, 
giving  rise  to  the  formation  of  free  alpha  ferrite  and  of  a  solution  of  carbon  in  alpha  iron  (troostite?). 
On  cooling  from  Ar2  to  Ari  additional  alpha  ferrite  is  liberated  while  the  proportion  of  carbon-alpha 
iron  solution  decreases  correspondingly.  At  An  the  remaining  solution  has  become  of  eutectoid 
composition  and  is  converted  bodily  into  pearlite,  the  mechanism  of  this  transformation  being  well 
understood.  It  will  be  evident  that  in  the  case  of  hypo-eutectoid  steel  having  but  one  upper  critical 
point,  Ars.2,  in  cooling  through  that  point  the  metal  would  pass  from  the  condition  of  a  solid  solution 
of  carbon  in  gamma  iron  to  that  of  a  solid  solution  in  beta  iron  and  then  immediately  to  that  of  a 
solid  solution  in  alpha  iron,  the  steel  between  Ar3.2  and  Arj  being  composed  of  this  solution  in  alpha 
iron  (troostite?)  and  of  free  alpha  ferrite.  With  eutectoid  steel  the  following  changes  would  take 
place  as  it  cools  through  its  single  critical  point  Ar3.2.i:  (1)  transformation  of  gamma  iron  solid  solu- 
tion (austenite)  into  beta  iron  solid  solution  (martensite?);  (2)  immediately  followed  by  the  forma- 
tion of  alpha  iron  solid  solution  (troostite?) ;  (3)  immediately  followed  by  formation  of  pearlite. 

The  author  thinks  that  the  decreasing  intensities  of  the  points  A3  and  A2  as  the  carbon  content 
increases  is  the  fact  most  difficult  to  reconcile  with  the  hypothesis  just  outlined,  for  if  these  points 
are  due  to  allotropic  transformations  affecting  the  entire  bulk  of  the  steel  their  intensities  should  be 
quite  independent  of  the  amount  of  carbon  present.  To  be  sure,  this  gradual  diminution  of  the 
magnitude  of  the  points  A3  and  A2  as  the  carbon  increases  is  likewise  difficult  of  explanation  in  the 
light  of  the  universally  accepted  hypothesis  that  free  iron  only  can  be  allotropically  transformed,  for 
it  has  been  made  clear  in  the  foregoing  pages  that  these  points  must  indicate  then  the  beginnings  of 
transformations  and  not  transformations  carried  to  completion  at  those  critical  points,  so  that  the 


LESSON   VIII  — THE   THERMAL   CRITICAL   POINTS   OF  STEEL 


15 


in  tensities  of  the  points  should  be  little  affected  by  the  magnitude  of  the  transformations  themselves, 
that  is,  by  the  amount  of  free  ferrite  undergoing  allotropio  transformation  or,  which  is  the  same  thing, 
by  the  percentage  of  carbon  in  the  steel. 

The  view  just  outlined  as  to  the  mechanisms  of  the  allotropic  changes  is  further  depicted  dia- 


ii^a»a^^ 


Sa:i::v::i-!::L;:;::j 


grammatically  in  Figure  7,  in  which  the  critical  points  are  represented  as  covering  certain  ranges 
of  temperature  making  it  possible  to  show,  graphically,  the  changes  taking  place  within  these 
ranges.  Taking  an  iron  carbon  alloy  having,  for  instance,  the  composition  a  (some  0.20  per  cent 
carbon),  the  diagram  shows  that  above  Ar3  it  is  made  up  of  aa',  i.e.  of  100  per  cent  austenite;  on  cool- 
ing through  Ar3  it  is  gradually  converted  into  martensite;  between  Ar3  and  Ar2  beta  ferrite  is 


16  LESSON  VIII  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 

liberatedj  in  passing  through  Ar2  the  remaining  martensite  is  gradually  converted  into  a  solid  solu- 
tion of  carbon  and  alpha  iron  (troostite?)  while  the  free  beta  ferrite  is  converted  into  free  alpha 
ferrite;  between  Ar2  and  An  additional  alpha  ferrite  is  liberated ;  in  cooling  through  An  the  remain- 
ing solid  solution  (troostite?),  now  of  eutectoid  composition,  is  converted  into  pearlite.  The  struc- 
tural changes  occurring  in  steel  having  but  one  upper  critical  point  and  in  steel  of  eutectoid 
composition  are  similarly  depicted.  This  diagram  is  reproduced  from  the  "Journal  of  the  Iron 
and  Steel  Institute,"  No.  IV  for  1906,  Plate  LII. 


Examination 

Describe  and  discuss  the  theories  that  have  been  suggested  to  account  for  the 
critical  points  of  steel  A3,  A2,  Ai,  A3.2,  A3.2.i,  and  Acm. 


LESSON  IX 

THE  THERMAL  CRITICAL  POINTS   OF   IRON  AND   STEEL 

THEIR  EFFECTS 

o 
'•It  has  been  shown  in  preceding  lessons  that  the  thermal  critical  points  of  iron 

and  steel  are  due  chiefly  if  not  wholly  to  allotropic  transformations  of  the  iron.  It  is 
a  well-known  fact  that  when  a  substance  undergoes  an  allotropic  transformation 
many  of  its  properties  undergo  likewise  deep  and  sudden  changes  at  the  critical 
temperatures.  Color,  crystallization,  dilatation,  conductivity  both  for  heat  and  elec- 
tricity, strength,  ductility,  hardness,  specific  gravity  are  properties  frequently  affected 
as  a  body  passes  from  one  allotropic  form  to  another.  We  should  expect,  therefore, 
such  changes  to  take  place,  as  iron  undergoes  its  allotropic  transformations,  if  not  in 
all,  at  least  in  some  of  the  above  properties.  And  it  is  because  such  changes  do  take 
place  that  a  clear  understanding  of  the  occurrence  and  significance  of  the  critical 
points  is  of  much  practical  importance  to  the  iron  and  steel  metallurgist. 

CHANGES  AT  A3 

It  has  been  shown  that  the  point  A3  occurs  in  carbonless  iron  and  in  steel  con- 
taining less  than  some  0.35  per  cent  of  carbon  and  that  it  is  universally  believed  that 
this  point  indicates  an  allotropic  change,  the  iron  passing  from  the  gamma  to  the 
beta  condition  on  cooling  at  Ar3,  and  vice  versa,  from  the  beta  to  the  gamma  condi- 
tion on  heating  at  Acs.  It  should  be  borne  in  mind,  however,  as  fully  explained  in 
Lesson  VIII  that  the  general  belief  is  that  free  ferrite  only  undergoes  this  change. 
As  the  metal  cools  past  its  point  Ar3  the  following  abrupt  changes  in  some  of  its 
properties  have  been  noted. 

Dilatation.  —  The  metal,  which  above  the  point  Ar3  was  contracting,  as  is  the 
general  rule  with  all  cooling  bodies,  on  passing  through  the  point  Ar3  undergoes  sud- 
denly a  marked  dilatation,  amounting  to  over  jcW  of  its  length,  immediately  fol- 
lowed again  by  a  normal  contraction.  Such  dilatation  implies  that  the  change  of 
gamma  into  beta  iron  takes  place  with  augmentation  of  volume,  or  in  other  words 
that  gamma  iron  is  denser,  has  a  higher  specific  gravity  than  beta  iron.  The  dilata- 
tions occurring  at  Ar3  in  the  case  of  steels  containing  respectively  0.05  and  0.15  per 
cent  carbon  are  shown  graphically  in  Figure  1.  On  heating,  at  Ac3  a  spontaneous 
contraction  occurs  of  the  same  magnitude  as  the  dilatation  on  cooling.  Had  we  no 
other  evidence  of  an  allotropic  transformation  of  the  iron  'at  this  critical  tempera- 
ture, this  sudden  dilatation  taking  place  as  it  does  in  pure  iron  would  justify  our 
belief  in  its  existence. 

Electrical  Conductivity.  —  Above  the  point  A3  the  metal  has  an  electrical  resis- 
tance some  ten  times  greater  than  its  resistance  at  ordinary  temperature.  As  it  cools 
from  a  high  temperature  to  the  point  Ar3  there  is  but  a  feeble  decrease  of  its  electrical 

1 


2        LESSON  IX  — THE  THERMAL  CRITICAL  POINTS  OF  IRON  AND   STEEL 

resistance,  but  as  soon  as  Ars  is  reached  it  begins  abruptly  and  sharply  to  decrease 
and  keeps  on  decreasing  at  a  normal  rate  to  atmospheric  temperature.  At  A3,  there- 
fore, we  have  a  sudden  and  marked  change  in  the  variation  of  the  electrical  conduc- 
tivity corresponding  to  a  sharp  break  in  the  curve  expressing  the  relation  between 
temperature  and  electrical  resistance  as  shown  in  Figure  2.  On  heating,  at  Ac3  the 
opposite  change  takes  place,  that  is,  the  electrical  resistance  quite  suddenly  ceases  to 
increase.  So  marked  and  sudden  a  change  in  a  physical  property  is  in  itself  a  proof 
of  an  allotropic  transformation. 

Crystallization.  —  It  has  been  shown  in  Lesson  II  that  while  gamma  and  beta 
iron  both  crystallize  in  the  cubic  system  (Osmond)  octahedra  are  the  prevailing  form 
of  gamma  iron  while  the  cube  is  the  crystalline  form  of  beta  iron,  and  that  the  trans- 


^OO°          4OO'  6OO°  <3OO°          /OOO"  //OO' 

Fig.  1.  —  Dilatation  curves  of  various  carbon  steels. 

formation  of  gamma  into  beta  iron  includes  a  change  in  the  planes  of  symmetry,  at 
least  of  carburized  iron  (Osmond).  As  already  mentioned,  however,  the  crystallo- 
graphic  differences  between  gamma  -and  beta  iron  are  not  such  as  to  prove  the  exis- 
tence of  these  two  allotropic  varieties  of  iron. 

Hardness,  Ductility,  Strength.  —  Evidences  will  be  offered  later  to  show  that  as 
the  metal  passes  through  the  point  Ar3  the  iron  becomes  harder,  stronger,  and  less 
ductile;  in  other  words  that  gamma  iron  is  softer,  more  ductile,  and  weaker  than 
beta  iron. 

Dissolving  Power  for  Carbon.  —  Above  the  point  A3  iron  possesses  dissolving 
power  for  carbon,  while  according  to  some  writers  it  loses  that  power  on  passing 
through  Ars ;  in  other  words  gamma  iron  can  dissolve  carbon  but  beta  iron  is  deprived 
of  that  power.  It  does  not,  however,  seem,  by  any  means,  proven  that  beta  iron 
cannot  dissolve  carbon,  many  authoritative  workers  holding  the  opposite  view.  This 
question  will  be  discussed  at  greater  length  in  another  lesson. 


LESSON   IX  — THE   THERMAL   CRITICAL   POINTS   OF   IRON   AND   STEEL         3 

Structural  Properties.  —  It  has  been  explained  at  length  in  Lesson  VIII  that 
the  point  Ar3  corresponds  to  an  abrupt  structural  change,  namely,  the  beginning  of 
the  setting  free  of  ferrite  (Fig.  1,  Lesson  VIII). 

Other  Properties.  —  Le  Chatelier  mentions  a  change  in  the  variation  of  the 
thermo-electric  force  and  a  sudden  but  slight  variation  in  magnetic  properties  as 
taking  place  at  As. 

CHANGES  AT  A2 

As  a  separate  point  A2  occurs  in  carbonless  iron  and  in  steel  containing  less  than 
some  0.35  per  cent  of  carbon.  The  changes  of  properties  taking  place  at  A2  are  not 
generally  as  abrupt  nor  are  they  as  marked  as  those  occurring  at  the  other  critical 
points.  It  is  precisely  because  of  this  lack  of  sharpness  and  suddenness  that  some 
metallographists,  notably  Le  Chatelier,  have  questioned  the  accuracy  of  the  gen- 
erally accepted  view  that  this  point  like  A3  indicates  an  allotropic  transformation. 
Careful  consideration  of  the  evidences  at  hand  appear  to  show,  however,  that  changes 


O°        <?OO°     4OO°      <SOO°      QOO°      /OOO° 
Fig.  2.  —  Electrical  resistance  curves  of  iron  and  high  carbon  steel. 


of  properties  do  occur  at  A2  sufficiently  marked  and  sudden  to  warrant  the  classifica- 
tion of  this  point  as  an  allotropic  one.  The  fact  that  these  changes  are  more  gradual 
than  at  the  other  critical  points  is  logically  explained  by  Osmond  on  the  ground  that 
beta  and  alpha  iron  are  isomorphous,  that  is,  capable  of  forming  solid  solutions  and 
that  therefore  the  passage  of  one  variety  into  the  other  must  necessarily  be  gradual 
as  well  as  the  variations  of  the  properties  of  iron  which  the  transformation  implies. 

Dilatation.  —  According  to  Le  Chatelier,  to  Charpy  and  Grenet,  and  to  some 
others,  no  dilatation  takes  place  as  the  metal  cools  past  the  point  Ar2  and  they  see  in 
this  an  indication  that  A2  is  not  an  allotropic  point.  Osmond's  reply  is  that  the 
curves  obtained  by  Charpy  and  Grenet,  for  instance,  do  indicate  a  dilatation  at  Ar2 
which,  however,  the  authors  fail  to  notice  because  the  transformation  not  being 
sudden  the  dilatation  likewise  is  gradual,  whereas  the  authors  were  looking  for  sud- 
den dilatations  only.  If  according  to  Osmond  iron  does  expand  on  passing  through 
the  point  Ar2  the  specific  gravity  of  beta  iron  must  be  greater  than  that  of  alpha  iron. 

Magnetic  Properties.  —  Above  the  point  A2  steel  is  non-magnetic,  it  is  not  at- 
tracted by  a  magnet,  but  in  passing  through  Ar2  it  suddenly  becomes  strongly  mag- 
netic. Is  not  this  abrupt  and  momentous  change  alone,  in  the  magnetic  properties  of 


LESSON   IX  — THE  THERMAL  CRITICAL  POINTS  OF  IRON  AND  STEEL 


a  metal,  sufficient  proof  of  an  allotropic  transformation?  It  is  true  that  magnetism 
is  not  fully  regained  until  a  considerably  lower  temperature  is  reached,  probably 
some  550  deg.  C.,  according  to  Osmond,  but  the  fact  remains  that  the  greatest  part  of 
the  final  magnetism  of  the  metal  is  abruptly  acquired  as  it  cools  past  the  point  A2. 
What  transformation  other  than  an  allotropic  one  can  satisfactorily  account  for  this? 

The  relation  between  the  carbon  content  of  steel  and  the  temperatures  at  which 
the  metal  loses  its  magnetism  on  heating  and  regains  it  on  cooling  is  shown  graphically 
in  Figure  3.  The  points  plotted  in  this  diagram  represent  the  average  values  of  a  great 
number  of  determinations  made  by  Madame  Sklodowska  Curie  with  a  series  of  very 
pure  carbon  steels.  It  will  be  noticed  that  the  points  of  magnetic  changes  correspond 
closely  to  the  thermal  critical  points  A2,  A3.2,  or  A3.2.i.  With  little  carbon  there  is  but 
a  small  gap  between  the  appearance  of  magnetism  on  cooling  and  its  disappearance  on 
heating,  because  the  points  Ar2  and  Ac2  occur  at  nearly  the  same  temperature;  with 
0.50  per  cent  carbon  the  magnetic  points  are  lowered  and  so,  likewise,  the  point  A3.2 
while  the  gap  increases,  this  being  consistent  with  the  greater  gap  between  Ar3.2  and 
Ac3.2;  with  0.84  per  cent  carbon  the  magnetic  points  are  further  lowered  and  the  gap 
between  them  increased  still  more,  this  being  in  harmony  with  the  location  of  the 
point  A3.2.i  which  is  lower  than  A3.2  and  with  the  greater  gap  between  Ar3.2.i  and  Ac3.2.i. 

Further  experimental  evidences  that  the  points  of  magnetic  transformations  coin- 
cide with  the  thermal  critical  points  are  given  in  the  following  tables  showing  the  re- 
sults of  several  hundred  determinations.  All  the  steels  used  in  connection  with  the 
results  given  in  Table  II  contained  in  the  vicinity  of  one  per  cent  carbon. 

TABLE  I.  —  COMPARISON  OF  THE  MAGNETIC   METHOD  WITH  THE 
ORDINARY   OR   COOLING-CURVE   METHOD.     (BOYLSTON.) 


Acs.j.t 

APPROXIMATE 
CARBON  CONTENT 
OF  STEEL 

METHOD 

NUMBER  OF 
TESTS 

a«i 

NUMBER  OF 
TESTS 

MEAN 

MAX. 
MIN. 

MKAX 

MAX. 

Mix. 

Magnetic  .... 

773 

{788 
1759 

7 

708 

{711 
I  706 

7 

1.25% 

Ordinary   .... 

764 

{781 
1754 

4 

718 

{736 

hoi 

4 

Ac,,.., 

Ar3, 

Magnetic  .... 

780 

{790 
1770 

7 

741 

{750 

1734 

7 

0.40% 

Ordinary   .... 

827 

{830 
1822 

3 

754 

{756 
1753 

3 

Ac2 

Ar, 

Magnetic  .... 

764 

{772 
1755 

6 

764 

(771 
1758 

7 

0.15% 

Ordinary   .... 

768 

{772 
1764 

2 

767 

[783 
1748 

3 

LESSON   IX  — THE   THERMAL   CRITICAL   POINTS   OF   IRON   AND   STEEL       5 
TABLE    II. —RESULTS    OBTAINED    BY    STUDENTS    AT    HARVARD    UNIVERSITY 


STEEL 
NUMBER 

METHOD 

Acj.,., 

NUMBER  OF 
TESTS 

Ar3.2., 

NUMBER  OF 
TESTS 

Magnetic  .... 

753 

72 

679 

75 

Ordinary   .... 

739 

14 

688 

12 

Magnetic  .... 

752 

131 

695 

136 

Ordinary   .... 

750 

13 

695 

17 

Magnetic  .... 

761 

55 

704 

55 

Ordinary   .... 

757 

55 

70S 

55 

Magnetic  .... 

762 

50 

7QQ_ 

50 

Ordinary   .... 

751 

45 

701 

45 

Magnetic  .... 

776 

40 

720 

40 

Ordinary   .... 

760 

30 

727 

30 

Crystallization.  —  The  cube  being  the  crystalline  form  both  of  beta  and  of  alpha 
iron  and  these  two  allotropic  varieties  being  capable  of  dissolving  each  other  in  all 


Fig.  3.  —  Temperatures  of  magnetic  transformations  of  various  carbon  steels. 

proportions  (Osmond)  a  crystalline  transformation  at  the  point  A2  is  not  to  be  ex- 
pected. The  crystallography  of  iron  so  far  as  it  has  been  investigated  does  not  re- 
veal the  existence  of  the  point  A2. 

Hardness,  Ductility,  Strength.  —  It  will  be  shown  in  another  lesson  that  as  the 
metal  passes  through  the  point  Ar2  it  becomes  softer,  more  ductile,  and  less  tenacious, 
in  other  words  that  beta  iron  is  harder,  stronger,  and  less  ductile  than  alpha  iron. 

Dissolving  Power  for  Carbon.  —  Most  metallographists  believe  that  alpha  iron 
does  not  possess  any  dissolving  power  for  carbon,  or  at  least  that  it  is  only  capable  of 
dissolving  a  very  small  amount  of  that  element.  On  the  other  hand,  as  already  men- 
tioned, it  is  far  from  certain  that  beta  iron  is  incapable  of  dissolving  carbon,  state- 
ments to  the  contrary  notwithstanding.  It  is  possible,  therefore,  that  on  cooling 
through  Ar2,  iron  loses  its  dissolving  power  for  carbon. 

Structural  Properties.  —  By  referring  to  Figure  1  of  Lesson  VIII  it  will  be  seen 
that  there  is  no  apparent  structural  change  connected  with  the  point  A2.  As  the  steel 
cools  past  Ar2  the  liberation  of  ferrite  started  at  Ar3  merely  continues,  to  end  only  at 
Ari.  Of  course  the  ferrite  liberated  above  Ar2  now  passes  from  the  beta  to  the  alpha 


6        LESSON   IX  — THE  THERMAL  CRITICAL  POINTS  OF   IRON  AND  STEEL 

condition  but  this  allotropic  transformation  does  not  appear  to  include  any  struc- 
tural change. 

Other  Properties.  —  Goerens  and  Cavalier  both  mention  a  sudden  decrease  in  the 
specific  heat  of  iron  as  taking  place  at  Ac2,  and  vice  versa  a  sudden  increase  at  Ar2. 

CHANGES  AT  A3.2 

It  has  been  shown  that  the  point  A3.2  resulting  from  the  merging  of  A3  and  A2 
occurs,  theoretically  at  least,  in  steels  containing  from  some  0.35  to  0.85  per  cent 
carbon.  As  might  be  expected  the  changes  of  properties  corresponding  to  the  point 
A3.2  are  the  same  as  those  taking  place  in  lower  carbon  steel  at  A3  and  A2.  As  the 
metal  cools  through  Ar3.2  the  following  variations  of  properties  are,  therefore,  noted: 
(1)  a  marked  dilatation,  (2)  a  sudden  decrease  of  electrical  resistance,  (3)  a  gain  of 
magnetism,  (4)  a  probable  loss  of  dissolving  power  for  carbon,  (5)  the  beginning  of 
the  liberation  of  alpha  ferrite  (see  Lesson  VIII,  Fig.  2). 

CHANGES  AT  AI 

The  point  AI  occurs  in  steel  containing  from  a  mere  trace  to  0.85  per  cent  carbon. 
It  corresponds,  as  explained  in  Lesson  VIII,  to  the  transformation  of  the  residual 
austenite  (now  of  eutectoid  composition)  into  pearlite.  This  formation  of  pearlite 
implies  that  the  iron  contained  in  this  residual  austenite  (and  forming  about  85  per 
cent  of  its  bulk)  undergoes  on  cooling  through  Ari,  the  same  allotropic  changes  as 
those  affecting  the  free  ferrite  on  cooling  through  Ar3  and  Ar2  (or  Ar3.2).  It  follows 
from  this  that,  theoretically  at  least,  the  following  sudden  changes  of  properties 
should  be  noted  on  cooling  through  Ar^  (1)  a  dilatation  increasing  with  the  carbon 
content  and  being  maximum  with  0.85  per  cent  carbon  caused  by  the  allotropic 
transformation  of  the  iron,  (2)  increased  magnetism  because  of  the  transformation  of 
additional  non-magnetic  gamma  iron  into  magnetic  alpha  iron,  (3)  decreased  electrical 
resistance  because  of  additional  transformation  of  high  resistance  gamma  iron  into 
low  resistance  alpha  iron,  and  (4)  additional  loss  of  dissolving  power  for  carbon  be- 
cause of  the  formation  of  additional  alpha  iron. 

Of  the  above  changes  the  dilatation  only  has  been  conclusively  shown  to  occur 
and  to  increase  with  the  carbon  content  (Fig.  1).  The  point  AI  has  never,  to  the 
author's  knowledge,  been  connected  with  critical  variations  of  the  electrical  and 
magnetic  properties  of  steel,  but  on  purely  theoretical  ground  he  does  not  see  how  the 
conclusion  can  be  avoided  that  such  critical  variations  must  exist  provided  of  course 
that  we  are  right  in  assuming  that  in  austenite  of  eutectoid  composition  the  iron  is 
still  in  the  gamma  condition,  that  is,  non-magnetic  and  of  high  electrical  resistance. 
It  is  possible  that  if  experiments  were  conducted  with  a  view  of  detecting  these  vari- 
ations, the  results  would  confirm  these  theoretical  deductions. 

CHANGES  AT  A3.2.i 

The  point  A3.2.i  occurs  in  eutectoid  and  in  hyper-eutectoid  steel  and  marks  the 
transformation  of  the  austenite  of  eutectoid  steel  or  of  the  residual  austenite  of  hyper- 
eutectoid  steel,  into  pearlite,  as  shown  in  Figures  3  and  4,  Lesson  VIII.  In  these 
steels,  however,  no  liberation  of  free  ferrite  occurs  above  the  point  Ar3.2.i  from  which 
it  follows  that  the  totality  of  the  iron  undergoes  its  allotropic  change  or  changes  on 
passing  through  Ar3.2.i.  The  variations  of  the  properties  which  in  hypo-eutectoid 


LESSON   IX  — THE   THERMAL   CRITICAL   POINTS   OF   IRON   AND   STEEL        7 

steel  occur  at  A3,  A2,  and  AI  must  therefore,  in  the  case  of  eutectoid  and  hyper- 
eutectoid  steel  all  take  place  at  the  point  A3.2.i.  These  sudden  changes  of  properties 
are,  on  cooling:  (1)  a  marked  dilatation,  maximum  in  eutectoid  steel  (Fig.  1),  (2)  a 
sudden  decrease  of  electrical  resistance,  (3)  a  sudden  gain  of  magnetism,  (4)  a  loss 
of  dissolving  power  for  carbon. 

Le  Chatelier  mentions  the  fact  that  on  cooling  through  Ari  or  Ar3.2.i  steel  ac- 
quires a  temporary  malleability.  If  a  steel  bar,  for  instance,  of  sufficient  length  be 
held  horizontally  by  one  extremity  while  cooling,  it  at  first  remains  rigid,  but  on 
passing  through  its  point  of  recalescence  it  quite  suddenly  bends. 

CHANGES  AT  Acm 

The  point  Arcm  occurs  in  hyper-eutectoid  steel  and  marks  the  beginning  of  the 
liberation  of  free  cementite  as  the  metal  cools  from  Arcm  to  Ar3.2.i  (see  Fig.  3,  Les- 
son VIII).  Except  for  this  structural  change  no  other  marked  changes  of  properties 
have  so  far  been  connected  with  this  point. 

Structural  Change  at  A!  and  A3.2.i.  —  The  spontaneous  transformation  of  aus- 
tenite  of  eutectoid  composition  into  pearlite,  that  is,  of  a  solid  solution  into  an  aggre- 
gate, at  Ari  or  Ar3.2.i  and  the  reverse  transformation,  from  aggregate  to  solid  solution, 
at  Aci  or  Ac3.2.i,  imply  structural  changes  of  momentous  importance  to  the  steel 
metallurgist.  While  these  will  be  dealt  with  at  length  in  another  lesson  it  seems 
proper  to  record  here  their  significance.  These  structural  changes  give  the  key  to  the 
rational  treatment  of  steel.  They  make  possible  the  refining  of  steel  by  heat  treatment 
seeing  that  on  heating  steel  through  its  critical  range  we  may  change  it  from  the 
condition  of  a  coarse  aggregate  (a  coarse  structure)  to  the  condition  of  a  fine,  nearly 
amorphous,  solid  solution.  They  also  make  possible  the  hardening  of  steel  through 
sudden  cooling  from  above  the  critical  range  as  will  be  fully  explained  in  another  lesson. 

Prevailing  Conditions  Above  and  Below  the  Critical  Range.  —  The  following 
condensed  statement  of  some  of  the  most  significant  conditions  prevailing  above 
and  below  the  critical  range  of  iron-carbon  alloys  may  be  useful  in  keeping  these 
fundamental  facts  in  mind.  By  critical  range  is,  of  course,  meant  here  the  critical 
points,  both  on  heating  and  cooling,  considered  collectively  and  it  will  be  understood 
that  the  -conditions  described  as  existing  above  the  range  change  quite  abruptly  to 
the  conditions  prevailing  below  the  range  as  the  metal  cools  through  the  range,  or 
vice  versa  as  it  is  heated  above  it.  The  references  made  to  the  crystallizing  of  the 
metal  and  to  the  influence  of  work  both  above  and  below  the  range  will  be  under- 
stood after  reading  the  following  lessons  dealing  with  the  treatment  of  steel. 

CONDITIONS  AND  PROPERTIES  OF  IRON-CARBON  ALLOYS  AND  OF  THEIR  CONSTITUENTS 

Above  Critical  Range  Below  Critical  Range 

Solid  solution  (austenite).  Aggregate  (ferrite  +  cementite). 

Hardening  (dissolved)  carbon.  Cement  carbon  (Fe3C). 

Gamma  iron.  Alpha  iron. 
Alloys  containing  a  sufficient  amount  of         Same  alloys  deprived  of  hardening  power. 

carbon  possess  hardening  power. 

Alloys  are  non-magnetic.  Alloys  are  magnetic. 

Metal  crystallizes  on  slow  cooling.  Metal  does  not  crystallize  on  slow  cooling. 

AVork  prevents  crystallization.  Work  distorts  structure. 


8        LESSON   IX  — THE   THERMAL   CRITICAL   POINTS   OF   IRON   AND   STEEL 


Properties  of  Gamma,  Beta,  and  Alpha  Iron.  —  The  various  properties  of  gamma, 
beta,  and  alpha  iron  described  in  the  preceding  pages,  have  been  tabulated  below  as 
well  as  some  other  data  of  interest.  These  are  in  accordance  with  the  views  most 
generally  held,  but  the  author  is  well  aware  that  those  entertaining  different  views 
may  take  exception  to  some  of  the  entries. 


GAMMA  IRON 

BETA  IRON 

ALPHA  IRON 

Metallurgical  name 

Austenite 

Beta  ferrite 

Ferrite,    alpha    ferrite, 

pearlite  ferrite 

Solvent  power  for  C  (or 

dissolves  carbon  up   to 

probably  some  but  opin- 

probably    none,      but 

Fe3C) 

1.7  per  cent  or  Fe3C  up 

ions  differ 

opinions  differ 

to  25.5  per  cent 

Range    of    temperature 

above  A3,  A3.2,  or  A3.2.i  in 

as  free  beta  ferrite  be- 

below A2,  Aa.2,  or  A3.2.i 

in  which  stable 

case    of    pro-eutectoid 

tween  As  and  A2 

ferrite,    above    Ai    or 

A3.2.i  in  case  of  eutec- 

toid  ferrite 

System    of    crystalliza- 

cubic (orthorhombic  ac- 

cubic 

cubic 

tion 

cording   to   Le   Chate- 

lier) 

Prevailing       crystalline 

octahedra 

cubes 

cubes 

forms 

Other   crystalline   char- 

frequent twinnings 

no  twinnings 

no  twinnings 

acteristics 

Specific  gravity 

greater  than   beta   and 

greater  than  alpha  iron 

smaller    than    gamma 

alpha   iron    (dilatation 

(gradual   dilatation   at 

and  beta  iron 

at  Ar3) 

Ar2,  Osmond) 

Electric  conductivity 

ten  times  smaller  than 

greater  than  that  of  gam- 

greater   than    that    of 

that  of  alpha  iron  at 

ma  iron  and  increasing 

beta  iron  and  increas- 

ordinary temperature 

with    falling    tempera- 

ing with  falling  tem- 

ture 

perature 

Magnetic  properties 

non-magnetic 

feebly  magnetic 

strongly  magnetic 

Hardness 

softer    than    beta    iron, 

very  hard 

soft 

harder  than  alpha  iron 

Examination 

I.  Describe  and  discuss  the  change  of  properties  occurring  in  steel  containing 
0.25  per  cent  carbon  as  it  cools  from  a  temperature  exceeding  its  critical 
range  to  ordinary  temperature. 

II.    Describe  the  changes  of  properties  taking  place  at  Ar3  2 1. 


LESSON  X 

CAST  STEEL 

The  structural  and  other  changes  taking  place  at  the  thermal  critical  points  of 
steel  account  for  the  deep  changes  of  properties  resulting  from  the  treatments  to 
which  steel  is  subjected  in  the  process  of  manufacture  of  steel  objects.  We  are  now 
in  a  position  to  understand  these  changes,  to  anticipate  them,  and  to  arrive  at  the 
rationale  of  the  treatment  of  steel  which  for  so  many  centuries  remained  purely  empir- 
ical. 

It  is  logical  that  we  should  first  consider  the  structure  of  steel  before  it  has  re- 
ceived any  treatment  whatsoever,  namely,  the  structure  of  the  metal  in  its  cast  con- 
dition. To  this  study  the  present  lesson  will  be  devoted. 

The  structure  of  cast  steel  is  different  from  what,  in  these  lessons,  has  been  termed 
the  normal  structure  of  the  metal  because,  having  been  developed  during  very  slow 
and  undisturbed  cooling  from  the  molten  condition,  crystalline  growth  has  been 
promoted,  whereas  in  working  and  reheating  such  large  growth  is  hindered  or  cor- 
rected. It  may  well  be  expected  then  that  the  structure  of  steel  castings  will  be 
coarser,  as  is  generally  expressed,  that  is,  made  up  of  larger  crystalline  grains,  than 
the  normal  structure  so  far  considered,  and  therefore  that  steel  castings  will  suffer 
from  all  the  ills  that  pertain  to  a  coarse  structure,  namely,  weakness,  lack  of  ductility, 
or  even  brittleness,  etc.  It  will  be  profitable  at  the  outset  to  consider  in  a  general 
way  the  genesis  of  the  structure  of  cast  eutectoid,  hypo-eutectoid,  and  hyper-eutectoid 
steel. 

Structure  of  Cast  Eutectoid  Steel.  —  Let  us  first  look  into  the  genesis  of  the  struc- 
ture of  eutectoid  steel  in  the  cast  condition.  Above  its  melting-point  this  steel,  like  all 
steels,  consists  of  a  liquid  solution  of  carbon  or  of  the  carbide  Fe3C  in  iron  (see  Fig.  4, 
Lesson  VIII).  Upon  solidifying  this  liquid  solution  is  converted  into  a  solid  solution, 
that  is,  the  carbon  or  carbide  remains  dissolved  in  the  iron,  known  now  as  gamma 
iron.  This  solid  solution  of  iron  or  carbide  of  iron  in  gamma  iron  is  called  austenite. 
Solidification  means  crystallization:  crystals  or  crystallites  of  austenite  form  during 
the  solidification  and,  as  is  usual,  the  slower  the  solidification  the  larger  will  the  crys- 
tals be.  Osmond  has  shown  that  these  crystals  belong  to  the  cubic  system  and  that 
they  are  chiefly  octahedra  (although  this  is  doubted  by  Le  Chatelier).  These  octa- 
hedra  of  austenite  continue  to  grow  on  slow  cooling  below  the  solidification,  this  growth 
being  effected  through  several  adjacent  crystalline  grains  taking  the  same  orientation 
and  therefore  merging  into  a  larger  crystalline  grain.  It  is  evident,  therefore,  that  in 
the  process  of  making  steel  castings  the  slow  and  undisturbed  cooling  prevailing  both 
during  and  after  solidification  promotes  the  formation  of  large  grains  of  austenite, 
these  grains  being  made  up  of  small  octahedric  crystals,  and  that  the  larger  the  cast- 
ings the  larger  generally  the  crystalline  grains,  that  is,  the  coarser  the  structure. 
When  slowly  and  undisturbedly  cooled  eutectoid  steel  then  reaches  its  single  critical 

1 


2  LESSON  X  — CAST  STEEL 

point,  As.2.1,  it  is  composed  of  relatively  large  grains  of  austenite.  In  passing  through 
this  point  the  austenite  grains  are  converted  bodily  into  as  many  pearlite  grains,  as 
explained  in  Lesson  VIII,  a  coarse  austenitic  structure  acquired  at  a  high  temperature 
giving  rise  to  a  coarse  pearlitic  structure  at  ordinary  temperature.  The  polyhedric 
structure,  therefore  (Fig.  1),  observed  after  complete  cooling  indicates  the  original  poly- 
hedric structure  of  austenite  formed  above  the  critical  point.  The  meshes  of  the  net- 
work are  sections  through  pearlite  grains,  the  net  merely  boundary  lines  between  such 
grains,  originally  boundary  lines  between  austenite  grains.  This  polyhedric  structure 
of  slowly  cooled  steel  proves  the  polyhedric  structure  of  austenite  at  a  high  tempera- 
ture. Because  of  a  coarser  grain  and  coarser  pearlite  cast  eutectoid  steel  is  weaker 
and  less  ductile  than  eutectoid  steel  properly  worked  or  annealed  or  both. 

Structure  of  Cast  Hypo-Eutectoid  Steel.  —  Let  us  now  consider  the  genesis  of  the 
structure  of  hypo-eutectoid  steel,  and  let  us  select  as  an  example  steel  containing 


Pig.  1.  —  Eutectoid  steel.    Cast.    Magnified 
500  diameters.     (Boylston.) 


Fig.  2.  —  Hypo-eutectoid  steel.  Cast.  Free  f en-it  o 
rejected  chiefly  to  the  boundaries.  Magnified 
100  diameters.  (H.  C.  Cridland  in  the  author's 
laboratory.) 


0.60  per  cent  carbon  and,  therefore,  composed  after  complete  slow  cooling  of  72  per 
cent  of  pearlite  and  28  per  cent  of  free  ferrite.  The  formation  of  the  structure  of 
this  steel  has  been  depicted  in  Figure  2,  Lesson  VIII,  to  which  the  reader  is  referred. 
This  steel  on  solidifying  passes,  like  all  steels,  from  the  condition  of  a  liquid  solution 
of  iron  and  carbon  to  that  of  a  solid  solution  of  carbon  (or  more  probably  Fe3C)  in 
gamma  iron,  this  solid  solution,  or  austenite,  being  made  up  of  crystalline  grains.  The 
austenite  grains  formed  during  solidification  continue  to  grow  as  the  steel  cools  slowly 
to  its  upper  critical  point  Ar3.2,  when,  as  explained  in  Lesson  VIII,  ferrite  begins  to  be 
liberated  and  continues  to  be  liberated  as  the  metal  cools  to  its  lower  point  Aiv  This 
setting  free  of  ferrite  is  apparently  brought  about  by  each  grain  of  austenite  rejecting 
the  ferrite  in  excess  of  the  eutectoid  composition,  so  that  by  the  time  the  point  Ari 
is  reached  each  residual  grain  of  austenite  has  the  eutectoid  composition  and  on  cooling 
through  Ari  is  converted  bodily  into  a  grain  of  pearlite.  Microscopical  examination 
reveals  the  fact  that  the  pro-eutectoid  ferrite  is  rejected  (1)  to  the  boundaries  of 


LESSON   X  — CAST  STEEL  3 

the  decreasing  austenitic  grains  and  (2)  between  the  cleavage  or  crystallographic 
pianos  of  these  crystalline  grains,  so  that  three  types  of  structures  may  be  distin- 
guished in  cast  hypo-eutectoid  steel,  (a)  structures  in  which  the  free  (pro-eutectoid) 
ferrite  has  been  rejected  chiefly  to  the  boundaries  of  the  austenitic  grains  (Fig.  2), 
clearly  indicating  that  these  grains  were  polyhedric,  (6)  structures  in  which  the  free 
ferrite  has  been  rejected  chiefly  between  the  cleavage  planes  of  austenite  (Fig.  3), 
proving  the  crystalline  character  of  that  constituent  and  suggesting  as  later  explained 
that  its  crystallization  is  cubic,  and  (c)  structures  in  which  the  free  ferrite  has  been 
rejected  partly  to  the  grain  boundaries  and  partly  between  the  cleavage  planes. 
Long  exposure  to  high  temperatures  followed  by  slow  cooling  _apj3ears  to  favor 
the  massing  of  free  ferrite  between  crystallographic  planes,  whereas  short  exposure 


Fig.  3.  —  Hypo-eutectoid  steel.  Cast.  Free  ferrite 
rejected  chiefly  between  cleavage  planes.  Mag- 
nified 100  diameters.  (W.  J.  Burger,  Corres- 
pondence Course  student.) 


and  more  rapid  cooling  promotes  the  expulsion  of  free  ferrite  to  the  grain  boundaries, 
resulting  in  sharply  defined  network  structures. 

The  structure  of  cast  hypo-eutectoid  steel  is  coarse  (1)  because  its  slow  and  un- 
disturbed cooling  promotes  the  formation  of  large  austenite  grains  and  hence,  later, 
of  large  pearlite  grains,  (2)  because  its  slow  cooling  between  the  upper  and  lower 
critical  points  favors  the  rejection  of  a  maximum  amount  of  free  ferrite  which  rejec- 
tion makes  for  coarseness  of  structure,  and  (3)  because  its  slow  cooling  from  the  upper 
critical  point  to  atmospheric  temperature  promotes  the  crystallization  of  free  ferrite 
into  large  grains,  this  influence,  however,  being  material  only  where  there  is  a  large 
amount  of  free  ferrite,  i.e.  in  very  low  carbon  steel. 

Because  of  its  coarser  structure  cast  hypo-eutectoid  steel  is  less  tenacious  and  less 
ductile  than  forged  or  properly  annealed  steel  of  similar  composition. 

Structure  of  Cast  Eutectoid  vs.   Structure  of  Cast  Hypo-Eutectoid    Steel.  — 
Although  the  pearlite  grains  of  eutectoid  steel  may  be  and  often  are  larger  than  the 
pearlite  grains  of  hypo-eutectoid  steel,  the  latter,  especially  when  judged  by  its  frac- 


4  LESSON  X  — CAST  STEEL 

ture,  is  the  coarser  of  the  two.  This  greater  coarseness  of  hypo-eutectoid  steel  in 
spite  of  smaller  pearlite  grains  is  due  to  the  presence  of  free  ferrite,  relatively  small 
pearlite  grains  surrounded  by  coarse  ferrite  envelopes  or  holding  coarse  ferrite  particles 
imparting  a  coarse  appearance  to  the  fracture  of  steel.  The  dimension  of  the  pearlite 
grains,  therefore,  while  not  without  influence,  is  not  the  criterion  by  which  to  judge 
of  the  coarseness  or  fineness  of  the  structure  and  fracture  of  hypo-eutectoid  steel,  the 
amount  of  free  ferrite  present  and  its  mode  of  distribution  having  to  be  taken  into 
consideration.  In  very  low  carbon  steel,  moreover,  there  is  but  little  pearlite  and 
the  small  amount  present  occurs  as  small  irregular  particles  (Fig.  4)  exerting  but  little 
influence  upon  the  character  of  the  fracture  which  now  depends  quite  exclusively  upon 
the  dimension  of  the  ferrite  grains.  As  ferrite  grains,  however,  no  matter  how  small 
never  impart  as  fine  a  structure  or  fracture  to  steel  as  pearlite  grains,  it  follows  that 


Fig.   4.  —  Hypo-eutectoid  steel.     Cast.     Carbon  0.20%. 
Magnified  28  diameters.     (Boylston.) 

low  carbon  (ferritic?)  steels  can  never  have  as  fine  a  structure  or  fracture  as  higher 
carbon  (pearlitic)  steels. 

Structure  of  Cast  Hyper-Eutectoid  Steel.  —  The  genesis  of  the  structure  of  cast 
hyper-eutectoid  steel  has  been  depicted  diagrammatically  in  Figure  3,  Lesson  VIII. 
Between  its  solidification  point  and  its  upper  critical  point  (Acm)  this  steel  is  com- 
posed, like  all  steels,  of  crystalline  austenite  grains  formed  on  solidifying  and  subse- 
quent slow  cooling.  Upon  reaching  the  point  Arcm  the  setting  free  of  cementite 
begins,  ending  only  at  the  lower  point  Ar3.2 1.  This  free  cementite,  like  the  free  ferrite 
of  hypo-eutectoid  steel,  is  rejected  (1)  to  the  boundaries  of  the  diminishing  austenite 
grains  and  (2)  between  the  cleavage  planes  of  this  crystalline  austenite,  giving  rise 
to  the  three  types  of  structure  described  in  the  case  of  hypo-eutectoid  steel,  but  in 
which  free  ferrite  is  replaced  by  free  cementite,  namely,  (a)  structures  in  which  the  free 
cementite  is  found  chiefly  at  the  grain  boundaries,  (6)  structures  in  which  the  free 
cementite  is  chiefly  located  between  cleavage  planes,  and  (c)  structures  in  which  the 
free  cementite  is  partly  at  the  boundaries  and  partly  between  crystallographic 


LESSON   X  — CAST   STEEL  5 

planes  (Fig.  5).  Like  the  structure  of  cast  hypo-eutectoid  steel,  the  structure  of 
cast  hyper-eutectoid  steel  bears  witness  (1)  to  the  polyhedric  form  of  the  austenite 
grains,  (2)  to  the  crystalline  character  of  these  grains,  and  (3)  to  their  probable  cubic 
crystallization. 

Long  exposure  to  high  temperatures  followed  by  very  slow  cooling  promotes  in 
hyper-eutectoid  steel  the  rejection  of  cementite  to  the  cleavage  planes,  while  short 
exposure  and  more  rapid  cooling  favor  the  rejection  of  cementite  to  the  boundaries. 

The  rejection  of  free  cementite  like  the  rejection  of  free  ferrite  makes  for  coarseness 
of  structure  and  fracture,  from  which  it  follows  that  cast  and  slowly  cooled  hyper- 
eutectoid  steel  will  be  coarser  than  cast  and  slowly  cooled  eutectoid  steel,  and  that 
the  more  free  cementite  it  contains,  that  is,  the  higher  the  carbon,  the  coarser  it  will 
be.  The  structure  and  fracture  of  hyper-eutectoid  steel,  however,  will  generally  be 


Fig.  5.  —  Hyper-eutectoid  steel.  Cast.  Free  cementite  rejected 
partly  to  the  boundaries  and  partly  between  cleavage  planes. 
Magnified  114  diameters.  (Boylston.) 

decidedly  finer  than  that  of  hypo-eutectoid  steel  because  of  the  very  small  amount  of 
free  cementite  present  in  the  former  compared  to  the  amount  of  free  ferrite  in  the  latter, 
unless,  indeed,  the  hypo-eutectoid  steel  be  very  near  the  eutectoid  composition.  This 
is  due  to  the  fact,  now  well  understood,  that  starting  from  the  eutectoid  composition 
(carbon  0.85  per  cent),  as  the  carbon  decreases  the  amount  of  free  ferrite  increases 
rapidly,  while  as  the  carbon  increases  above  the  eutectoid  ratio  the  amount  of  free 
cementite  increases  slowly  and  remains  small  even  with  high  carbon  content.  Steel 
with  0.50  per  cent  carbon,  for  instance,  contains  40  per  cent  of  free  ferrite,  hence 
its  coarseness  both  of  structure  and  fracture,  while  steel  with  say  1.25  per  cent  carbon 
contains  but  6.4  per  cent  of  coarsening  free  cementite,  hence  the  relative  fineness  of 
both  its  structure  and  fracture. 

Ingotism.  —  Howe  has  suggested  the  term  "ingotism"  to  designate  the  structure 
of  cast  steel  described  in  the  foregoing  pages  and  characterized  (1)  by  large  pearlite 
grains  and  (2)  by  coarse  ferrite  or  cementite  membranes  surrounding  them  and  by 
irregular  masses  of  these  constituents  located  in  some  of  the  cleavage  planes  of  the 
original  austenite  grains. 


6  LESSON   X  — CAST   STEEL 

Structure  of  Cast  Steel  vs.  Structure  of  Meteorites.  —  The  structure  of  meteo- 
rites thro^  'additional  light  upon  the  mechanism  of  the  formation  of  the  structure 
of  cast  stei  -,*,  outlined  in  the  foregoing  pages.  The  most  characteristic  feature  of  the 
structure  o>  hypo-  and  hyper-eutectoid  cast  steel,  namely,  the  coarse  massing  of  free 
ferrite  or  ceuentite  at  the  boundaries  of  the  austenite  grains  or  between  the  cleavage 
planes  of  the  crystalline  austenite  is  exhibited  most  strikingly  in  the  structure  of 
some  meteorites  known  as  the  Widmanstatten  structure.  This  structure,  however, 
is  on  a  much  larger  scale  than  the  similar  structure  of  steel  castings;  which  should 


Fig.  6.  —  Steel.     Carbon  0.55%.     Widmanstatten  structure.     Magnified  6 
diameters.     (Belaiew.) 

not  be  a  source  of  surprise  when  it  is  considered  that  the  conditions  required  for  the 
formation  of  such  structure  are  greatly  exaggerated  and  intensified  during  the  cool- 
ing of  meteorites,  namely,  (1)  a  very  slow  solidification  peroid,  (2)  very  long  exposure 
to  high  temperatures,  and  (3)  very  slow  cooling  from  these  high  temperatures. 

Belaiew  succeeded  in  a  remarkable  manner  in  reproducing  the  Widmanstatten 
structure  by  subjecting  carbon  steels  to  a  high  temperature  for  a  very  long  time  and 
cooling  them  extremely  slowly,  the  fall  of  temperature  from  1500  to  300  deg.  C.  last- 
ing 60  hours,  an  evident  attempt  at  reproducing  the  conditions  which  must  prevail 
during  the  solidification  and  further  cooling  of  meteorites.  The  structures  obtained 
by  Belaiew  in  the  case  of  steel  containing  0.55  per  cent  carbon  and  otherwise  of  com- 


LESSON   X  — CAST   STEEL  7 

mercial  quality  are  shown  in  Figures  6,  8,  10,  12,  and  13.  They  ar  >ical  struc- 
tures of  steel  castings  of  the  same  grade  but  on  a  much  larger  scale  should  be 
noted  that  the  magnification  of  Figures  8,  10,  12,  and  13  is  only  30  eters  while 
Figure  6  is  magnified  but  6  diameters.  Figure  6  is  a  beautiful  illus  tion  of  that 


Fig.  7.  —  Section  parallel  to  the  surface 
of  a  cube.     (Tschermak). 


Fig.  8.  —  Steel.     Carbon  0.55%.     Section  parallel  to  the 
surface  of  a  cube.     Magnified  30  diameters.     (Belaiew.) 

type  of  structure  in  which  the  free  ferrite  has  been  rejected  both  to  the  grain  boun- 
daries forming  a  sharply  outlined  network  and  between  crystallographic  planes. 
In  Figures  8,  10,  12,  and  13  the  free  ferrite  is  seen  massed  between  cleavage  planes. 

Octahedric  Crystallization  of  Austenite.  —  It  will  be  noted  that  the  ferrite  bands  shown  in  Figures 
8,  10,  12,  and  13  cut  each  other  at  right  angles  or,  more  frequently,  form  equilateral  triangles.  Ac- 
cording to  crystallographers  these  are  indications  that  austenite  crystallizes  in  regular  octahedra. 
That  this  inference  is  correct  appears  to  be  conclusively  proven  by  the  following  remarks  of  Belaiew. 


8 


LESSON  X  — CAST  STEEL 


"  Let  us  consider  an  octahedron  and  let  us  assume  that  four  systems  of  lamellae  locate  themselves 
in  this  octahedron  along  its  crystallographic  planes,  that  is,  parallel  to  the  four  pairs  of  its  surfaces, 
a  fact  that  has  been  long  known  in  the  case  of  meteoric  irons. 

"If  we  now  examine  any  section  of  the  octahedron,  we  shall  find  that  not  only  the  angles  formed 
liy  the  projections  of  the  lamellae  vary  in  different  sections,  but  that  the  number  itself  of  different 


Fig.  9.  —  Section  parallel  to  the  surface 
of  an  octahedron.     (Tschermak.) 


Fig.  10.  —  Steel.     Carbon  0.55%.    Section  parallel  to  tho  sur- 
face of  an  octahedron.     Magnified  30  diameters.     (Belaiew.) 

sections  varies  likewise  from  two  to  four.  For  instance,  when  the  section  is  parallel  to  the  surface 
of  the  cube,  the  number  of  different  sections  is  minimum,  that  is,  two,  and  in  the  entire  section  we 
find  only  two  systems  of  lamellae  forming  right  angles.  Figure  7  is  a  diagram  of  such  section 
and  Figure  8  a  corresponding  section  of  the  steel. 

"  A  section  parallel  to  one  of  the  surfaces  of  the  octahedron  will  yield  equilateral  t  riangles  formed 
by  three  systems  of  lamellae  forming  60°  angles;  the  fourth  system  coincides  with  the  section  con- 
sidered (see  Figs.  9  and  10). 


LESSON    X  — CAST   STEEL 


g 


"  In  a  section  parallel  to  the  surface  of  the  dodecahedron,  two  systems  of  lamella;  are  observed 
forming  an  angle  of  109°  28'  16";  the  other  two  systems  coincide  and  divide  this  angle  in  half 
(Pigs.  11  and  12).  Finally  any  section  will  give  four  different  systems  cutting  each  other  at  differ- 
ent angles  (Fig.  13). 

"All  these  cases,  as  we  have  just  seen,  can  very  well  be  illustrated  by  different  samples  of  our 


Fig.  11.  —  Section  parallel  to  the  surface 
of  a  dodecahedron.  Magnified  30  di- 
ameters. (Tschermak.) 


Fig.  12.  —Steel.     Carbon  0.55%.  Section  parallel  to  the  sur- 
face of  a  dodecahedron.     Magnified  30  diameters.     (Belaiew.) 

alloy  which,  firstly,  affords  a  rather  weighty  proof  of  the  octahedric  crystallization  of  steel  and,  sec- 
ondly, brings  out  the  remarkable  analogy  of  this  structure  with  that  of  meteorites  and  warrants  us 
to  allude  to  the  synthesis  of  the  structure  so  called  of  Widmanstatten  .  .  .  this  structure  is  the  neces- 
sary consequence  of  the  uniform  orientation  of  the  elementary  octahedra  within  a  volume  of 
greater  or  less  dimension;  it  is,  therefore,  in  no  way  related  with  the  carbon  content  and  must  be 


10 


LESSON  X  — CAST  STEEL 


obtained  in  any  alloy  of  iron  and  carbon  whenever  the  conditions  are  favorable  to  the  formation  of 
that  structure. 

"Moreover,  in  practice  this  structure  is  met  (although  certainly  much  less  developed  than  in 
our  alloys)  every  time  that  the  metal  is  subjected  to  an  intense  heating  followed  by  slow  cooling  as 
is  the  case  with  cast  steel  or,  better  still,  with  burnt  or  overheated  steel. 

"The  very  brittleness  of  these  steels  may  be  due  to  a  certain  extent  to  the  uniform  orientation 
of  the  elementary  octahedra  which  are  the  cause  of  that  structure." 

Let  it  be  noted  that  in  the  case  of  meteorites  the  length  of  time  during  which  the  metal  is  main- 
tained at  a  high  temperature  is  so  long  that  generally  but  one  crystal  is  formed,  that  is,  all  the 
elementary  octahedra  formed  on  solidification  have  assumed  the  same  orientation.  In  steel  the 
conditions  being  less  favorable  to  uniformity  of  orientation  we  have  several  grains. 


?J 


/*/., 


Fig.  13.  —  Steel.     Carbon  0.55%.     Four  systems  of  lamellae. 
Magnified  30  diameters.      (Belaiew.) 


Experiments 

The  student  should  procure  samples  of  cast  steel  containing  the  following  pro- 
portions of  carbon:  from  0.20  to  0.50  per  cent,  from  0.70  to  0.90  per  cent,  and  1.25 
or  more  per  cent.  These  should  be  prepared  for  microscopical  examination  in  the 
usual  way,  and  examined  both  with  low  and  high  powers.  They  should  be  photo- 
graphed under  a  magnification  not  exceeding  100  diameters  as  the  aim  should  be  to 
bring  out  the  structural  characteristics  described  in  this  lesson,  for  which  purpose 
a  high  magnification  is  not  necessary  or,  indeed,  desirable.  In  the  case  of  the 
hypo-eutectoid  steel  several  specimens  may  be  prepared  and  examined  to  illustrate 
the  different  aspects  which  cast  steel  of  that  grade  may  assume  as  explained  in 
the  lesson. 

Examination 

Describe  briefly  the  genesis  of  the  structure  of  (1)  cast  hypo-eutectoid  steel,  (2) 
cast  eutectoid  steel,  and  (3)  cast  hyper-eutectoid  steel. 


LESSON  XI 
THE  MECHANICAL  TREATMENT  OF  STEEL 

Forged  steel  objects  are  manufactured  by  subjecting  the  cast  metal  (1)  to  pres- 
sure exerted  by  rolls,  presses,  or  dies,  or  to  blows  from  hammers  and  (2)  by  reheating 
it  to  various  temperatures  for  various  lengths  of  time  and  cooling  TE  at  various  rates. 
In  other  words  we  have  to  consider  (1)  the  mechanical  treatment  of  steel  and  (2)  its 
heat  or  thermal  treatment.  The  machining  of  steel  is  not  here  mentioned  since  it 
can  evidently  have  no  effect  upon  the  structure  and  properties  of  the  metal,  unless  it 
be  a  very  superficial  one. 

While  the  primary  purpose  of  working  steel  is  to  shape  it  into  useful  appliances 
and  while  the  primary  purpose  of  reheating  it  may  be,  and  often  is,  to  impart  to  the 
metal  such  plasticity  as  will  facilitate  its  being  so  shaped,  both  mechanical  and  heat 
treatments  deeply  affect  the  structure  of  steel  and  therefore  its  physical  properties. 
To  this  very  important  subject  the  next  four  lessons  will  be  devoted.  We  shall  first 
consider  the  influence  of  mechanical  treatment  and  then  that  of  heat  treatment. 

The  effect  of  work  upon  the  structure  and  properties  of  steel  greatly  depends 
upon  the  temperature  of  the  metal  while  it  is  being  worked,  the  expressions  "hot  work- 
ing" and  "cold  working"  being  common  ones,  the  former  meaning  working  the  steel 
while  hot,  and  the  second  working  it  while  cold,  more  specifically  at  atmospheric 
temperature.  In  these  lessons  the  expression  hot  working  will  be  applied  to  working 
the  metal  while  above  its  critical  range,  and  cold  working  to  work  performed  below 
that  range.  In  justification  of  this  course  it  will  be  shown  that  the  effect  of  work 
changes  quite  sharply  as  the  critical  temperature  of  steel  is  passed. 

Hot  Working.  —  Hot  working  may  be  applied  to  steel  (1)  after  it  has  solidified 
but  before  it  has  cooled  to  a  much  lower  temperature  so  that  it  still  possesses  the 
necessary  plasticity,  or  (2)  the  steel  ingot  may  be  allowed  to  cool  to  atmospheric  tem- 
perature or  at  least  to  a  temperature  so  low  that  reheating  is  required  as  a  preliminary 
step  to  successful  hot  working.  The  following  considerations  will  show  that  so  far  as 
the  influence  of  hot  work  is  concerned  it  is  quite  immaterial  whether  the  steel  ingot 
has  or  has  not  been  completely  cooled  before  being  brought  to  the  forging  tempera- 
ture. Let  us  assume  that  the  steel  ingot  be  allowed  to  cool  to  atmospheric  tempera- 
ture, that  it  is  then  reheated  to  a  temperature  well  above  its  critical  range  and  then 
subjected  to  hot  working. 

An  attempt  has  been  made  in  Figure  1  to  depict  graphically  the  influence  of  hot 
work  on  the  structure  of  steel.  The  diagram  will  be  readily  understood.  The  critical 
range  both  on  heating  and  cooling  is  represented  by  a  double  line,  but  the  reader 
will,  of  course,  bear  in  mind  that  the  critical  range  on  heating  does  not  coincide  with 
the  range  on  cooling  and  that  each  range  may  include  one,  two,  or  three  critical  points, 
as  fully  explained  in  previous  lessons.  For  the  present  purpose  it  is  preferable  to 
represent  both  ranges  irrespective  of  the  number  of  critical  points  included  by  the 
two  parallel  lines  shown  in  the  diagram.  The  solidification  range  is  likewise  indi- 
cated by  a  double  line.  The  widths  of  the  shaded  areas  are  intended  to  be  propor- 

1 


LESSON   XI  — THE  MECHANICAL  TREATMENT  OF  STEEL 


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LESSON   XI  — THE   MECHANICAL  TREATMENT  OF  STEEL  3 

tional  to  the  grain  size  resulting  from  the  various  treatments;  when  an  area  is  reduced 
to  a  mere  line  the  corresponding  grain  is  very  small,  i.e.  the  structure  very  fine.  As 
the  steel  ingot  solidifies  at  A  crystalline  grains  are  formed  which  increase  in  size  on 
slow  and  undisturbed  cooling  to  the  critical  range  B.  In  passing  through  that  range 
the  austenite  grains  are  converted  into  pearlitic  grains  with  or  without  rejection  of 
free  ferrite  or  of  free  cementite  according  to  the  carbon  content  of  the  steel.  On 
cooling  from  B  to  C  there  is  no  further  growth  and  the  size  of  the  final  grain  may  be 
represented  by  the  width  of  the  shaded  area  at  C.  The  metal  has  now  the  usual 
coarse  structure  of  steel  castings  described  in  Lesson  X.  Upon  reheating  this  coarsely 
crystalline  steel  ingot  from  S  to  R  and  through  its  critical  range,  i±  is.  converted  from 
the  condition  of  an  aggregate  of  ferrite  and  cementite  into  a  nearly  amorphous  solid 
solution,  so  that  as  the  metal  emerges  from  its  range  it  has  a  very  fine  structure. 
As  steel  just  above  its  critical  range,  however,  would  not  be  plastic  enough  for  hot 
working  and  would  not  afford  the  necessary  range  of  temperature  through  which  to 
cool  while  being  worked  and  still  remain,  as  it  should,  above  the  range,  it  is  generally 
necessary  to  heat  the  metal  to  a  much  higher  temperature,  say  to  some  1200  deg.  C. 
or  even  more  (W  in  Fig.  1).  As  the  metal  is  heated  from  its  critical  range  to  this 
high  temperature,  it  probably  crystallizes  so  that  when  work  begins  at  w  it  is  in  a 
somewhat  crystalline  condition.  This  grain  growth  on  heating  from  R  to  W  is  de- 
picted in  the  diagram.  The  heavy  pressure  or  blows  which  are  now  applied,  how- 
ever, soon  break  up  this  crystallization  while  preventing  a  new  one  from  forming,  so 
long  at  least  as  the  work  continues  sufficiently  vigorous,  for  undisturbed  cooling  is  a 
condition  necessary  for  the  ready  growth  of  crystals.  As  soon  as  work  ceases,  how- 
ever, the  metal  being  now  left  to  cool  undisturbedly,  if  its  temperature  is  then  above 
its  critical  range,  for  instance  at  /,  it  will  immediately  begin  to  crystallize  and  its 
crystallization  will  continue  from  /  to  r,  that  is,  until  its  critical  range  is  reached,  when 
it  will  stop  and  the  austenite  grains  just  formed  will  be  converted  into  as  many  pearl- 
ite  grains  with  or  without  rejection  of  free  ferrite  or  free  cementite.  The  width  of 
the  band  at  s  is  intended  to  represent  the  size  of  the  final  grain.  It  is  smaller  than  the 
grain  of  the  steel  ingot  but  it  is  still  large. 

If  work  ceases,  then,  while  the  metal  is  still  above  its  critical  range  it  is  evident 
that  it  will  crystallize  and  that  the  resulting  grains  will  be  the  larger,  that  is,  its 
structure  will  be  the  coarser,  the  higher  the  temperature  at  which  the  work  was 
stopped. 

Finishing  Temperatures.  —  The  temperatures  at  which  work  ceases  are  known  as 
the  finishing  temperatures  and  the  above  considerations  will  show  the  importance  of 
proper  finishing  temperatures  if  it  be  desired  to  impart  to  steel  implements  the  finest 
grain  that  it  can  acquire  through  working  together  with  the  desirable  properties 
inherent  to  it. 

Should  hot  work  be  continued,  for  instance,  until  the  temperature  of  the  metal 
is  but  slightly  above  its  critical  range,  /'  in  Figure  1,  the  austenitic  grains  that  will 
form  on  undisturbed  cooling  to  the  range  will  be  small,  so  that  the  final  grain  of  the 
metal  at  .s'  will  likewise  be  small,  as  indicated  by  the  width  of  the  shaded  area.  If 
the  finishing  temperature  is  exactly  at  the  critical  range,  /",  the  final  grain  at  s"  will 
be  very  fine.  If  working  be  continued  until  the  metal  has  cooled  to  a  temperature 
lower  than  its  critical  range,  for  instance  /'"  in  the  diagram,  the  structure  will  be 
fine  since  crystallization  was  prevented  while  the  metal  was  cooling  above  its  critical 
range,  but  it  will  be  distorted  because  the  effect  of  working  below  the  range,  that  is, 


4  LESSON  XI  — THE   MECHANICAL  TREATMENT  OF  STEEL 

of  cold  working,  is  to  distort  the  structure  as  explained  in  the  following  pages.  A 
distorted  structure  in  turn  means  decreased  ductility  and  eventually  brittleness.  It 
seems  evident,  therefore,  that  hot  worked  steel  implements  should  be  finished  exactly 
at  their  critical  ranges  or  at  temperatures  but  slightly  superior  or  inferior  to  them. 

Structure  of  Hot  Worked  Eutectoid  Steel.  —  If  eutectoid  steel  be  subjected  to 
hot  work  until  it  has  nearly  reached  its  critical  range  (now  consisting  of  the  single 
point  Ars.2.i),  /"  in  Figure  1,  it  will  have  a  very  fine  austenitic  structure  which  in 
cooling  slowly  and  undisturbedly  through  the  critical  range  at  r"  will  be  converted 
into  a  fine  pearlitic  structure.  The  metal  will  then  have  as  fine  a  structure  as  can 
be  imparted  to  it  by  work  alone,  followed  by  slow  cooling  through  the  range. 

If  eutectoid  steel  be  worked  until  its  temperature  is  still  considerably  above  its 
critical  point  (/,  Fig.  1),  and  then  allowed  to  cool  undisturbedly,  austenite  grains  begin 


Fig.  2.  —  Hot  worked  hypo-eutectoid  steel. 
Carbon  0.50%.  Finishing  temperature  near 
critical  range.  Magnified  100  diameters. 
(Burger,  Correspondence  Course  student.) 

to  form  and  increase  in  size  as  the  metal  cools  to  its  critical  point,  i.e.  from  f  to  r 
when  these  austenitic  grains  are  converted  into  as  many  pearlitic  grains.  The  final 
grain  size  therefore  will  depend  upon  the  temperature  at  which  work  ceased  and  will 
be  the  greater  the  higher  that  temperature. 

Structure  of  Hot  Worked  Hypo-Eutectoid  Steel.  —  If  hypo-eutectoid  steel  be 
worked  until  its  temperature  is  but  very  slightly  above  its  critical  range  (/",  Fig.  1) 
and  then  allowed  to  cool  undisturbedly,  small  grains  of  austenite  are  formed  which 
on  passing  through  the  critical  range  at  r"  are  converted  into  pearlite  grains  with 
rejection  of  excess  free  ferrite  as  now  well  understood.  Because  of  the  small  size  of 
the  austenite  grains  and  because  of  relatively  quick  cooling  (in  air)  the  rejected  fer- 
rite generally  locates  itself  at  the  boundaries  of  the  grains  and  network  structures 
are  produced  (see  Fig.  2).  In  the  case  of  low  carbon  steel,  however,  containing  say 
less  than  0.30  per  cent  carbon  the  proportion  of  free  ferrite  is  so  large,  i.e.  the  ferrite 
net  so  thick,  that  the  structure  consists  of  particles  of  pearlite  embedded  in  a  matrix 
of  ferrite  (see  Fig.  3). 


LESSON   XI  — THE   MECHANICAL  TREATMENT   OF   STEEL  5 

If  tho  working  of  hypo-eutectoid  steel  ceases  at  a  temperature  considerably  above 
its  critical  range,  /  in  Figure  1,  the  austenite  grains  formed  on  cooling  to  the  range 
be  relatively  large  and  therefore  also  the  pearlite  grains  of  the  completely  cooled 


Fig.    3.  —  Hot   worked    hypo-eutectoid   steel.     Carbon 
0.05%.    Magnified  114  diameters.    (Boylston.) 


Fig.  4.  —  Hot  worked  hypo-eutectoid  steel.  Carbon 
0.50%.  Finishing  temperature  considerably  above 
the  critical  range.  Magnified  56  diameters. 

metal.  The  free  ferrite  will  still  be  found  chiefly  at  the  boundaries  (Fig.  4),  the 
meshes  of  the  network  structure,  however,  being  larger  than  in  similar  steel  finished 
at  a  lower  temperature,  as  will  be  apparent  from  a  comparison  of  Figures  2  and  4. 


6  LESSON   XI  — THE   MECHANICAL  TREATMENT   OF   STEEL 

It  will  be  seen  'that  in  forged  hypo-eutectoid  steel  forming  network  structures  on 
slow  cooling  through  the  critical  range  a  close  relation  must  exist  between  the  size  of 
the  meshes  and  the  finishing  temperature. 

Structure  of  Hot  Worked  Hyper-Eutectoid  Steel.  —  If  hyper-eutectoid  steel  be 
hot  worked  until  its  temperature  is  very  near  its  critical  range  (/",  Fig.  1)  and 
then  allowed  to  cool  undisturbedly,  small  austenite  grains  are  formed  which  on 
cooling  through  the  range  are  converted  into  small  pearlite  grains  with  rejection  of 
free  cementite,  as  shown  in  Figure  5.  If  the  work  be  stopped  at  a  temperature  con- 
siderably above  the  critical  range  (/,  Fig.  1),  the  final  pearlite  grains  will  be  larger 
while  the  free  cementite  will  be  located  chiefly  at  the  grain  boundaries,  a  network 
structure  being  produced.  It  will  be  evident  that  a  close  relation  exists  between 


Fig.  5.  —  Hot  worked  hyper-eutectoid  steel.  Carbon 
1.50%.  Finishing  temperature  near  critical  range. 
Magnified  100  diameters.  (Reinhardt  in  the 
author's  laboratory.) 

the  size  of  the  meshes  of  these  network  structures  and  the  corresponding  finishing 
temperatures. 

Sorbite.  —  High  magnification  of  the  structure  of  the  meshes  of  both  hypo-  and 
hyper-eutectoid  steel  described  in  the  foregoing  pages  often  fails  to  reveal  the  char- 
acteristic features  of  pearlite,  namely,  (1)  sharply  defined  parallel  plates  alternately 
of  ferrite  and  cementite  and  (2)  a  constant  or  nearly  constant  carbon  content.  The 
structure  of  these  meshes  instead  remains  indistinct  and  presents  a  granular  rather 
than  a  lamellar  aspect  (Fig.  6).  It  is  also  frequently  noted  in  connection  with  these 
network  structures  that  the  full  amount  of  free  ferrite  or  of  free  cementite  has  not 
been  rejected,  the  pearlitic  grains  having  retained  some  of  the  constituent  in  excess  of 
the  eutectoid  ratio.  To  this  imperfectly  developed  pearlite  the  name  of  sorbite  has 
been  given  (Osmond)  and  quite  universally  adopted  in  spite  of  recent  and  regrettable 
efforts  to  eliminate  it  from  metallographic  nomenclature.  It  will  be  apparent  that 
the  formation  of  sorbite  results  from  a  relatively  quick  cooling  through  the  critical 


LESSON   XI  — THE   MECHANICAL  TREATMENT  OF  STEEL  7 

range,  time  being  denied  for  the  crystallization  of  distinct  lamellae  of  ferrite  and 
cementite,  and,  in  the  cases  of  hypo-  and  hyper-eutectoid  steel,  for  the  rejection  of 
the  full  amount  of  free  ferrite  or  free  cementite.  The  cooling  in  air  of  hot  worked 
pieces,  especially  when  of  small  size,  is  often  sufficiently  rapid  to  cause  the  formation 
of  sorbite  rather  than  of  pearlite. 

The  production  and  nature  of  sorbite  will  be  dealt  with  at  greater  length  in  an- 
other lesson.  It  should  be  mentioned  here,  however,  that  while  sorbite  is  less  ductile 
than  pearlite  it  has  a  higher  tenacity,  higher  elastic  limit,  and  greater  hardness  (hence 
greater  wearing  power).  When  these  qualities  are  required,  in  hot  forged  objects, 
they  may  consequently  be  obtained,  although  necessarily  at  the~  sacrifice  of  some 


Fig.  6.  —  Hypo-eutectoid  steel.    Carbon  0.50%.    The  ill-defined  constituent  is 
sorbite.     Magnified  1000  diameters.     (Boynton.) 

ductility  and  softness,  by  hastening  the  cooling  through  the  critical  range,  after  work 
has  ceased,  when  sorbitic  rather  than  pearlitic  steel  will  be  produced. 

Hot  Working  of  Steel  vs.  Its  Critical  Range.  —  In  conducting  the  hot  working  of 
steel  so  as  to  impart  to  the  metal  the  finest  grain  that  can  result  from  finishing  at 
suitable  temperatures,  it  is  generally  necessary  only  to  consider  its  lower  critical 
point  on  cooling,  namely,  Ari  or  Ars.a-i.  The  following  considerations  will  justify 
this  statement.  In  the  case  of  hypo-eutectoid  steel  if  the  working  be  continued  while 
the  metal  cools  from  its  upper  point  or  points  to  its  lower  point,  it  will  make  for  fine- 
ness of  structure  by  preventing  a  coarse  massing  of  the  free  ferrite  while  the  cold 
working  of  that  constituent,  if  taking  place  at  all,  must  be  very  slight.  The  same 
reasoning  applies,  with  greater  force,  to  the  hot  working  of  hyper-eutectoid  steel  from 
its  upper  point  (denoting  the  formation  of  free  cementite)  to  its  lower  point  Ar3.2.i. 
Greater  fineness  of  structure  will  result  with  very  little,  if  any,  distortion  of  the  free 
cementite. 

It  is  apparent,  therefore,  that  in  order  to  secure  the  finest  grain  obtainable  through 


8  LESSON   XI— THE   MECHANICAL  TREATMENT   OF   STEEL 

mechanical  refining  without  appreciable  structural  distortion  steel  objects  should  be 
finished  slightly  above  their  lower  critical  point,  that  is,  in  the  vicinity  of  700  deg.  C. 
for  all  grades  of  commercial  carbon  steel. 

Cold  Working.  —  By  the  cold  working  of  steel  is  meant  in  these  pages  the  work- 
ing of  it  while  its  temperature  is  below  its  critical  range.  It  will  now  be  shown  that 
the  effect  of  cold  working  upon  the  properties  of  the  metal  is  very  different  from  that 
of  hot  working.  This  should  not  be  a  cause  for  surprise  if  it  be  borne  in  mind  that 
steel  above  its  critical  range  is  in  a  condition  totally  different  from  its  condition 
below  it.  Above  the  critical  range  we  have  to  deal  with  a  solid  solution  of  iron  and 
carbon,  below  it  with  an  aggregate  of  ferrite  and  cementite.  The  solid  solution  exist- 
ing above  the  range  will  crystallize  if  allowed  to  cool  undisturbedly  and  it  has  been 
shown  in  the  foregoing  pages  that  working  in  this  range,  i.e.  hot  working,  is  effective 


Fig.  7.  —  Cold  worked  hypo-eutectoid  steel. 
Carbon  0.30%.  Magnified  100  diameters. 
(Burger,  Correspondence  Course  student.) 

in  preventing  or  at  least  retarding  this  crystallization,  thus  imparting  a  smaller  grain 
to  the  metal.  The  aggregate  of  ferrite  and  cementite  existing  below  the  range,  on 
the  contrary,  exhibits  no  tendency  to  crystallize  during  slow  and  undisturbed  cooling, 
because  this  aggregate  was  formed  and  fully  developed  while  passing  through  the 
range,  the  size  of  its  elements,  that  is,  its  coarseness  depending  (1)  upon  the  coarse- 
ness of  the  solid  solution  immediately  before  its  transformation  and  (2)  upon  the 
time  occupied  in  cooling  through  the  range.  Working  this  aggregate,  therefore,  as  it 
cools  to  atmospheric  temperature,  or  working  it  while  at  atmospheric  temperature, 
i.e.  cold  working  steel,  does  not  prevent  its  crystallization.  Its  effect  consists  in  dis- 
torting the  existing  aggregate  structure,  chiefly  through  the  stretching  or  elongation 
of  its  crystalline  elements  (free  ferrite,  free  cementite,  pearlite)  in  the  direction  of  the 
forging,  and  such  distortion  in  turn  means  decreased  ductility  and  eventually  extreme 
brittleness.  The  effect  of  cold  working  upon  the  structure  of  steel  is  illustrated  in 
Figures  7  and  8  in  the  case  of  hypo-eutectoid  steel.  It  is  also  depicted  in  the  diagram 
of  Figure  1.  While  the  structural  distortion  caused  by  cold  working  is  very  slight 
near  the  critical  range  of  the  metal,  it  rapidly  increases  as  the  temperature  decreases, 


LESSON   XI  — THE   MECHANICAL   TREATMENT   OF   STEEL 


g 


becoming  very  pronounced  at  atmospheric  temperature.  The  manufacture  of  wire  by 
cold  drawing  affords  a  familiar  instance  of  the  effect  of  work  performed  at  atmos- 
pheric temperature  both  on  the  structure  and  properties  of  the  metal.  It  is  well 
known  that  after  the  wire  has  been  passed  through  several  dies  it  becomes  so  brittle 
that  annealing  is  necessary  in  order  to  make  further  reduction  in  size  possible,  the 
annealing  operation  removing  the  structural  distortion  and  brittleness  produced  by 
working  at  atmospheric  temperature. 

Mechanical  Refining.  —  It  would  seem  as  if  with  the  use  of  pyrometers  at  least  it 
should  be  a  relatively  simple  matter  to  finish  steel  objects  very  near  the  desirable 


Fig.  8.  —  Cold  worked  hypo-eutectoid  steel.    Carbon  0.30%.     Magnified 
150  diameters.     (Buck,  Correspondence  Course  student.) 


temperature  and  thus  secure  for  them  the  best  structure  that  can  be  imparted  by 
work  alone.  Upon  reflection,  however,  it  will  be  manifest  that  the  problem  is  on  the 
contrary  an  insoluble  one,  for  the  reason  that  unless  the  objects  are  of  very  small 
cross-sections,  it  is  quite  impossible  to  finish  them  so  that  their  temperature  will  be 
uniform  throughout,  the  central  portions  being  necessarily  hotter  than  the  outside. 
Should  the  forging  be  so  conducted  that  the  temperature  of  the  outside  be  very  near 
the  critical  range,  the  center,  being  materially  hotter,  will  coarsen  on  cooling,  while  if 
the  implements,  on  the  contrary,  are  finished  so  that  their  center  may  have  the  fine 
structure  produced  by  ceasing  the  work  at  the  proper  temperature,  their  outside  must 
necessarily  suffer  from  cold  working.  The  limitations  of  work  alone  as  a  means  of 
imparting  the  best  possible  structure  to  steel  are  therefore  quite  evident. 

While  a  uniformly  fine  grain  cannot  be  imparted  to  steel  objects  of  considerable 
size  through  hot  working  alone,  the  value  of  hot  work  as  a  means  of  refining  the 
structure  of  steel  remains  very  great  as  exemplified  by  the  structure  of  properly  hot 
forged  steel  when  compared  with  the  structure  of  steel  castings  of  similar  composi- 
tion. The  finer  grain  imparted  to  steel  by  working  it  has  been  called  by  some  writers 


10  LESSON   XI  — THE   MECHANICAL  TREATMENT   OF   STEEL 

"mechanical"  refining  to  distinguish  it  from  the  refining  produced  by  heat,  i.e.  from 
"thermal"  refining.  In  practise  hot  work  should  be  so  conducted,  that  is,  the  finish- 
ing temperatures  so  regulated,  that  the  central  portions  of  the  finished  implements 
will  not  suffer  unduly  from  the  coarsening  influence  of  too  high  a  finishing  tempera- 
ture, while  at  the  same  time  the  outside  will  not  suffer  unduly  from  the  effect  of  cold 
working.  The  natural  tendency  of  rolling  and  other  forging  mills  is  to  finish  work 
at  too  high  temperatures  for  the  simple  reason  that  the  metal  is  then  more  plastic 
and  consequently  requires  less  power  for  its  working.  In  some  manufactures,  how- 
ever, and,  especially  in  the  rolling  of  rails,  the  importance  of  proper  finishing  tempera- 
tures has  been  given  careful  attention  and  the  rolling  operation  so  modified  as  to 
deliver  rails  of  much  finer  grain  and  therefore  better  physical  quality,  than  formerly. 
Besides  its  important  grain  refining  influence  hot  work  further  improves  the  quality 
of  steel  by  closing  and,  if  the  carbon  be  low  enough,  welding,  the  blow-holes  and 
otherwise  increasing  its  soundness  and  by  removing  cooling  strains. 


Experiments 

The  student  should  prepare  for  microscopical  examination  samples  of  hot  forged 
hypo-eutectoid,  eutectoid,  and  hyper-eutectoid  steel.  Their  structure  should  be  com- 
pared with  the  structure  of  similar  steels  in  the  cast  condition  (Lesson  X)  and  also 
with  the  structure  of  similar  normalized  steels  (Lessons  IV  and  V).  If  the  forging  of 
these  samples  was  finished  at  a  fairly  high  temperature  their  structure  should  be 
quite  similar  to  the  normal  structure  of  like  steels  described  in  Lessons  IV  and  V. 
Finer  structures  point  to  lower  finishing  temperatures. 

A  sample  of  cold  worked  steel  preferably  the  longitudinal  section  of  an  unannealed 
cold  drawn  wire  should  likewise  be  prepared  and  its  structural  distortion  noted. 

If  a  cross-section  of  a  rail  or  of  some  other  rolled  shape  can  be  obtained,  a  piece 
should  be  cut  from  the  center  and  one  near  the  edge  (advisably  also  from  the  web 
and  extremity  of  flange  in  the  case  of  a  rail  section).  These  pieces  should  be  pre- 
pared and  examined  and  the  coarser  grain  of  the  central  portion  noted. 

All  specimens  should  be  photographed  preferably  under  a  magnification  not  ex- 
ceeding 100  diameters. 

Examination 

Describe  briefly  the  effect  upon  the  structure  of  steel  (1)  of  working  above  the 
critical  range  (hot  work)  and  (2)  of  working  below  that  range  (cold  work). 


LESSON  XII 

THE  ANNEALING  OF  STEEL 

Purpose  of  Annealing.  —  The  purpose  of  annealing  steel  may  be  (1)  to  increase 
its  softness  and  ductility  that  it  may,  for  instance,  be  more  easily-  machined  or  (2)  to 
secure  a  desirable  combination  of  high  strength  and  elastic  limit  with  fair  ductility 
that  it  may  successfully  stand  the  strains  to  which  it  is  to  be  subjected.  These  changes 
of  physical  properties  result  from  corresponding  changes  in  the  structure  of  the  metal 
brought  about  by  proper  heat  treatment.  In  annealing  steel  it  is  generally  intended 
to  impart  to  it  as  fine  a  structure,  that  is  as  small  a  grain,  as  is  consistent  with  the 
nature  of  the  treatment  and  the  grade  of  the  steel.  Hot  forged  steel  objects  maybe 
improved  by  annealing  for  certain  purposes,  because  of  their  structure  being  often  (1) 
relatively  coarse  owing  to  high  finishing  temperature  and  (2)  heterogeneous  as  ex- 
plained in  Lesson  XI.  The  structure  of  cold  worked  steel,  at  least  when  severely  cold 
worked,  is  so  defective  that  the  metal  must  be  annealed  before  it  can  be  put  to  useful 
purposes.  Finally,  steel  castings  have  so  coarse  a  structure  as  to  be  very  deficient  both 
in  strength  and  ductility  and  should  always  be  refined  by  annealing. 

Nature  of  the  Annealing  Operation.  —  The  annealing  operation  comprises  three 
distinct  steps:  (1)  heating  the  steel,  (2)  keeping  its  temperature  constant  at  the  an- 
nealing temperature,  and  (3)  cooling  it  from  the  annealing  to  atmospheric  tempera- 
ture. These  steps  will  be  considered  separately. 

Heating  for  Annealing.  —  The  first  step  in  annealing  always  consists  in  heating 
the  metal  past  its  critical  range  because  by  so  doing  the  preexisting  structure,  how- 
ever coarse,  is  obliterated;  the  metal,  for  the  time  being,  assuming  a  nearly  amorphous 
structure.  This  important  structural  change  is  due,  as  we  now  understand  it,  to  the 
passage  of  the  steel  from  the  state  of  an  aggregate  of  ferrite  and  cementite  to  that  of 
a  homogeneous  solid  solution,  and  it  is  not  to  be  wondered  at  that  so  radical  a  struc- 
tural change  should  destroy  effectively  any  preexisting  crystallization.  The  annealing 
of  steel  castings,  however,  constitutes  an  apparent  exception  to  the  rule  that  heating 
just  through  the  range  is  sufficient  to  break  up  effectively  the  preexisting  structure, 
for  their  successful  annealing  often  requires  a  materially  higher  temperature.  Should 
the  temperature  of  the  steel  remain  below  its  critical  range,  no  structural  change 
would  take  place  and  the  annealing  would  be  ineffective  (I,  Fig.  I).1  Should,  on  the 
contrary,  the  temperature  of  the  steel  be  carried  considerably  above  the  range,  its 
structure,  which  was  finest  as  it  emerged  from  the  range,  begins  to  coarsen  on  further 
heating  and  continues  to  grow  as  the  metal  cools  slowly  to  the  range,  so  that  its  final 
structure  would  be  at  least  relatively  coarse  (II,  Fig.  1).  Clearly,  therefore,  to  an- 

1  AYhen  steel  contains  hardening  carbon  it  may  be  softened  and  made  more  ductile  by  heating 
it  to  temperatures  lower  than  its  critical  range  (as  in  the  tempering  of  hardened  steel)  but  such  treat- 
ment is  not,  or  at  least  should  not  be,  called  annealing.  Cooling  strains  may  also  be  removed,  at 
least  in  part,  by  heating  below  the  range. 

1 


LESSON  XII  — THE  ANNEALING  OF  STEEL 


neal  steel  forgings  they  should  be  heated  through  their  critical  range  and  kept  at  a 
temperature  as  close  to  the  upper  part  of  that  range  as  possible  (III,  Fig.  1).  The 
annealing  temperature  will,  of  course,  vary  with  the  carbon  content  since  the  position 
of  the  critical  range,  or  rather  its  width,  varies  likewise.  The  following  ranges  of  tem- 
peratures are  recommended  by  the  Committee  on  Heat  Treatment  of  the  American 
Society  for  Testing  Materials.  The  report  of  the  Committee  states  that  for  steels 
containing  more  than  0.75  per  cent  manganese  slightly  lower  temperatures  suffice. 


RANGE  OF  CARBON  CONTENT 
Less  than  0.12  per  cent 
0.12  to  0.25  per  cent 
0.30  to  0.49  per  cent 
0.50  to  1.00  per  cent 

17 


RANGE  OF  ANNEALING  TEMPERATURE 
875  to  925  deg.  C.  (1607-1697  deg.  F.) 
840  to  870  deg.  C.  (1544-1598  deg.  F.) 
815  to  840  deg.  C.  (1499-1.544  deg.  F.) 
790  to  815  deg.  C.  (1454-1499  deg.  F.) 


m 

r\ 

sr 
r\ 

Cr/r/co/ 

Rnnnf* 

/ 

\\ 

I 

n. 

\  V 

S 

il 

^    v      Vi 

5 

^ 

8    V     v 

N, 

1 

[9 

§.         \%         \n 

3j  ° 

O           \^           \o 

•Q 

0 

•Q  ^ 

\                  V" 

s 

IK  (-, 

i  O 

\                   \ 

i- 

Q| 

\                  \ 

d 

C* 

\                     \ 

, 

Structure                Coarse                              OAF                Very   f/ne    structure 

unchanged             sfruc/vre                           /~/ne       structure                      Strong,  e/ast/c,   and  tough 

O     Hardest,  strongest,  one/  /east  ducft/e 

F    Softest,  w&okest,  ond  most  ducf//e 

Fig.  1.  —  Diagram  depicting  the  annealing  of  steel. 

Time  at  Annealing  Temperature.  —  The  steel  object  should  be  kept  at  the  anneal- 
ing temperature  long  enough  to  be  heated  right  through  to  that  temperature.  The 
Committee  on  Heat  Treatment,  referred  to  above,  states  that  an  exposure  of  one  hour 
should  be  long  enough  for  pieces  twelve  inches  thick.  Thicker  pieces,  of  course,  need 
a  longer  heating. 

The  usefulness  of  pyrometers  in  conducting  annealing  operations  is  obvious. 
Their  use  is  to  be  strongly  recommended. 

Cooling  from  Annealing  Temperature.  —  Having  imparted  a  fine  structure  to  the 
steel  the  next  step  must  be  to  retain  it.  The  most  effective  way  of  accomplishing 
this  consists  in  cooling  the  steel  very  quickly,  by  quenching  it  in  water  for  instance, 
as  time  is  then  denied  for  the  structure  to  coarsen  at  all  while  the  metal  cools  to  at- 
mospheric temperature.  Such  rapid  cooling,  however,  as  is  well  known,  hardens  the 
metal  and  deprives  it  of  ductility  (unless,  indeed,  it  contains  very  little  carbon) ,  and 


LESSON  XII  — THE  ANNEALING  OF  STEEL  3 

this  would  defeat  the  purpose  of  annealing  which  always  demands  the  retention  of 
considerable  ductility.  It  follows  from  these  considerations  that,  in  annealing,  cool- 
ing from  the  annealing  temperature  cannot  be  so  rapid  as  to  very  materially  harden 
the  steel.  Its  rate  should,  moreover,  be  regulated  in  accordance  with  the  kind  of 
properties  we  most  desire  the  steel  object  to  possess.  For  instance,  (1)  if  softness 
and  ductility  are  wanted  (for  ease  in  machining) ,  necessarily  at  a  certain  sacrifice  of 
strength  and  elasticity,  the  cooling  should  be  very  slow,  to  wit,  with  the  furnace  in 
which  the  object  was  heated,  (2)  if  greater  hardness  (for  wearing  power),  strength, 
and  elasticity  are  desired,  at  the  necessary  sacrifice  of  some  ductility,  the  cooling 
should  be  more  rapid  as,  for  example,  in  air  or,  in  the  case  of  low  carbon  steel,  in  oil 
or,  with  very  low  carbon  steel,  even  in  water  (III,  Fig.  1). 

Rate  of  Cooling  vs.  Carbon  Content.  —  The  lower  the  carbon  content  the  more 
rapid  may  be  the  cooling  from  the  annealing  temperature  without  affecting  too  deeply 


Fig.  2.  —  Steel.  Carbon  0.50  per  cent.  Magni- 
fied 100  diameters.  Heated  to  1000  deg.  C. 
and  slowly  cooled  in  furnace.  (W.  J.  Burger, 
Correspondence  Course  student.) 

the  ductility  of  the  metal.  For  instance,  (1)  steel  containing  not  over  0.15  per  cent 
carbon  may  be  quenched  in  water,  thereby  increasing  its  strength  and  elastic  limit 
and  still  remain  very  ductile,  (2)  steel  with  less  than  0.20  or  0.30  per  cent  carbon  may 
be  quenched  in  oil  with  satisfactory  results,  (3)  with  a  larger  proportion  of  carbon 
such  rapid  cooling  is  no  longer  possible,  as  it  would  destroy  the  ductility  of  the  metal, 
recourse  having  then  to  be  had  to  cooling  in  air  for  the  desired  combination  of  strength 
and  ductility  or  to  the  double  annealing  treatment  soon  to  be  described. 

Rate  of  Cooling  vs.  Size  of  Object.  —  Since  large  objects  necessarily  cool  more 
slowly  than  smaller  ones  when  subjected  to  the  same  cooling  influences,  it  is  evident 
that  the  external  conditions  should  also  be  regulated  in  accordance  with  the  dimen- 
sions of  the  objects  treated.  To  secure  maximum  softness  and  ductility,  for  instance, 
the  cooling  of  small  objects  should  be  more  effectively  retarded  than  the  cooling  of 
larger  ones.  Assume,  for  example,  two  objects  made  of  the  same  steel,  one  large  and 
one  small,  and  both  cooled  in  air  from  the  annealing  temperature;  the  smaller  object 


4  -       LESSON   XII  — THE   ANNEALING   OF   STEEL 

will  be  harder  and  less  ductile  than  the  larger  one,  because  of  its  quicker  cooling.  To 
render  it  as  soft  and  ductile  as  the  larger  object  cooling  in  the  furnace  may  be  neces- 
sary. Similarly,  to  give  strength  and  high  elastic  limit  the  cooling  of  large  objects 
must  be  more  vigorously  hastened  than  that  of  smaller  objects  as,  for  instance,  cooling 
in  oil  against  cooling  in  air  for  the  smaller  piece. 

Furnace  Cooling  from  Annealing  Temperature.  —  As  an  example  of  the  effect  of 
furnace  cooling  upon  the  structure  of  steel,  let  us  take  a  steel  bar  J/£  in.  square,  con- 
taining 0.50  per  cent  carbon,  heated  to  1000  deg.  C.  and  slowly  cooled  with  the  fur- 
nace. Its  structure  is  shown  in  Figures  2  and  3.  It  will  be  seen  to  be  composed  of 
the  normal  proportions  of  pearlite  and  free  ferrite,  namely,  some  60  per  cent  of  the 
former,  and  it  will  also  be  noted  that  the  pearlite  is  distinctly  laminated  (Fig.  3), 


Fig.  3.  —  Steel.  Carbon  0.50  per  cent.  Magnified  670  diameters.  Heated 
to  1000  deg.  C.  and  slowly  cooled  in  furnace.  (C.  C.  Buck,  Correspondence 
Course  student.) 

and  that  in  places  at  least  the  ferrite  forms  characteristic  polyhedric  grains.  This 
structure  is  due  to  the  slow  cooling  of  the  steel  through  its  critical  range,  which  per- 
mits the  rejection  of  the  full  amount  of  free  ferrite  and  a  distinct  crystallization  of 
the  constituents  of  the  residual  austenite  into  plates  of  ferrite  and  cementite.  The 
relative  softness  and  great  ductility  of  the  steel  in  this  condition  is  due  (1)  to  the 
presence  of  the  full  amount  of  soft  ferrite  in  relatively  large  areas  and  (2)  to  the  pres- 
ence of  distinctly  laminated  pearlite  indicating  the  absence  of  hardening  carbon  as 
explained  later. 

Air  Cooling  from  Annealing  Temperature.  —  To  illustrate  the  influence  of  air 
cooling  upon  the  structure  of  steel,  let  us  take  likewise  a  steel  containing  some  0.50 
per  cent  carbon,  heated  to  1000  deg.  C.  and  cooled  in  air.  Its  structure  is  shown  in 
Figure  4.  It  will  be  found  quite  unlike  the  structure  of  the  same  steel  after 
furnace  cooling  (Figs.  2  and  3).  It  contains  a  much  smaller  proportion  of  free  ferrite, 
apparently  not  over  20  per  cent,  in  the  form  of  a  distinct  net  surrounding  dark  meshes 


LESSON   XII  — THE   ANNEALING   OF  STEEL  5 

which  a  high  magnification  fails  to  resolve  into  distinct  parallel  plates.  '-  Relatively 
quick  cooling  through  the  critical  range  has  prevented  the  separation  of  the  normal 
amount  of  free  ferrite,  from  which  it  necessarily  follows  that  the  dark  constituent 
contains  more  ferrite  than  true  pearlite;  nor  has  it  the  structure  of  true  pearlite,  time 
also  having  been  denied  on  cooling  through  the  range  for  the  formation  of  distinct 
plates  of  ferrite  and  cementite.  Sorbite  is  the  name  of  this  constituent.  The 
structure  of  pearlite  passing  into  sorbite  is  shown  in  Figure  5. 

Properties  of  Sorbite.  —  Sorbite  has  already  been  briefly  described  in  Lesson  XI, 
where  it  was  shown  that  it  could  be  produced  in  steel  forgings  of  small  sections 
through  simple  air  cooling  from  a  finishing  temperature  superior,  to  the  critical  range, 


Fig.  4.  —  Steel.  Carbon  0.50  per  cent.  Magnified  100 
diameters.  Heated  to  1000  deg.  C.  and  cooled  in 
air.  (W.  J.  Burger,  Correspondence  Course  student.) 

and  in  larger  sections  by  hastening  somewhat  their  cooling  through  that  range.  It 
has  also  been  stated  that  sorbite  is  harder,  stronger,  and  less  ductile  than  pearlite. 
By  so  regulating  the  cooling  from  the  annealing  temperature,  therefore,  that  sorbitic 
steel  is  produced,  hardness,  strength,  and  elasticity  will  be  promoted  at  the  sacrifice 
of  some  ductility  (III,  Fig.  1).  It  will  be  explained  in  another  lesson  that  sorbite  is 
generally  regarded  as  one  of  the  transition  stages  assumed  by  the  metal  as  it  passes 
from  its  austenitic  condition,  stable  above  the  critical  range,  to  its  pearlitic  condition, 
stable  below  that  range. 

Influence  of  Maximum  Temperature.  —  The  influence  of  the  maximum  tem- 
perature to  which  steel  is  heated  before  being  allowed  to  cool  is  well  shown  in 
Figures  6  to  9  which  should  be  compared  with  Figures  2  to  5.  They  refer  to  steel 
containing  0.50  per  cent  carbon  and  heated  to  800  deg.  C.,  while  the  structures 
illustrated  in  Figures  2  to  5  refer  to  the  same  steel  but  heated  to  1000  deg.  The 
constitutents  are  the  same,  namely,  ferrite  and  pearlite  in  the  furnace  cooled 
samples,  ferrite  and  sorbite  in  the  air  cooled  samples,  but  the  higher  temperature 
resulted  in  the  formation  of  larger  particles  of  pearlite  or  sorbite,  evidently  because 
of  the  formation  above  the  critical  range  of  larger  austenitic  grains. 


6  LESSON  XII  — THE  ANNEALING  OF  STEEL 

Influence  of  Time  at  Maximum  Temperature.  —  Maintaining  steel  for  a  long 
time  at  a  high  temperature  causes  the  formation  of  large  austenite  grains,  which  in 
passing  through  the  range  are  converted  into  large  pearlite  or  sorbite  grains 
with  rejection  of  free  ferrite  in  hypo-eutectoid  steel  and  of  free  cementite  in  hyper- 
eutectoid  steel.  It  is  also  noted  that  the  more  prolonged  the  heating  the  smaller 
the  amount  of  the  excess  constituent  (free  ferrite  or  free  cementite)  separating  on 
rapid  (air)  cooling  through  the  range.  This  is  shown  in  Figure  10  in  which  is 
depicted  the  structure  of  steel  containing  0.50  per  cent  carbon  heated  to  1150  deg.  C. 
for  two  hours  and  air  cooled.  The  very  large  sorbitic  grains  should  be  noted  as 
well  as  the  very  small  proportion  of  free  ferrite. 


Fig.  5. — -Steel.     Carbon  1.00  per  cent.      Magnified   1500  diameters. 
Pearlite  (laminated)  passing  into  sorbite.     (Osmond.) 

Oil  and  Water  Quenching  from  Annealing  Temperature.  —  As  already  explained 
only  steel  containing  very  little  carbon  may  be  quenched  in  oil  or  water  for  purposes 
of  annealing,  unless,  indeed,  the  double  treatment  soon  to  be  described  be  employed 
when  higher  carbon  steels  may  be  so  quenched.  The  structure  of  steel  containing 
0.10  per  cent  carbon,  heated  to  950  deg.  and  quenched  in  water,  is  shown  in  Figure  11, 
while  in  Figure  12  is  seen  the  structure  of  steel  containing  0.20  per  cent  carbon 
quenched  in  oil  from  a  temperature  of  850  deg.  Rapid  cooling  through  the  range  did 
not  prevent  the  separation  of  the  bulk  of  the  large  amount  of  excess  ferrite  present  in 
these  steels,  hence  their  softness  and  ductility  even  after  quenching.  They  are,  how- 
ever, somewhat  stronger  and  more  elastic  than  similar  steels  more  slowly  cooled,  (1) 
because  they  contain  a  somewhat  smaller  proportion  of  soft  free  ferrite,  (2)  because 
the  free  ferrite  they  contain  has  crystallized  into  smaller  grains,  and  (3)  because  their 
carburized  constituent  is  sorbitic  or  even  martensitic '  rather  than  pearlitic. 

1  Martensite  is  the  ordinary  constituent  of  steel  hardened  by  quenching.  It  is  hard  and  de- 
prived of  ductility. 


Fig.  6.  —  Magnified  100  diameters.     Heated  to 
800  deg.  C.  and  slowly  cooled  in  furnace. 


Fig.  8.  —  Magnified  100  diameters.    Heated  to 
800  deg.  C.  arid  cooled  in  air. 


Fig.  7. —  Magnified  670  diameters.    Heated  to         Fig.  9.  —  Magnified  670  diameters.     Heated  to 
800  deg.  C.  and  slowly  cooled  in  furnace.  800  deg.  C.  and  cooled  in  air. 

Figs.  6-9. —  Steel.    Carbon  0.50  per  cent.     (C.  C.  Buck,  Correspondence  Course  student.) 

7 


8 


LESSON  XII  — THE  ANNEALING  OF  STEEL 


Double  Annealing  Treatment.  —  It  has  been  stated  that  the  most  effective  way 
of  retaining  in  the  cold  the  very  fine  structure  acquired  by  steel  in  passing  through 
its  critical  range  consisted  in  cooling  it  very  rapidly  as  soon  as  it  emerged'  from  that 
range,  as,  for  instance,  by  quenching  it  in  water.  This  treatment,  however,  unless  the 
metal  contains  very  little  carbon,  hardens  the  steel  and  deprives  it  of  ductility,  where- 
as' annealed  steel  should  not  be  very  hard  and  should  possess  much  ductility.  If 
this  fine  grained  but  hard  steel,  however,  be  reheated  to  a  temperature  close  to  but 
below  its  critical  range,  say  to  from  500  to  650  deg.  C.,  it  loses  its  hardness  but  re- 
tains its  fine  structure  and  again  becomes  ductile  (IV,  Fig.  1). 

The  double  treatment  outlined  above  fulfils  admirably  the  aims  generally  sought 


Fig.  10.  —  Steel.      Carbon  0.50   per  cent.      Magnified   100  diameters. 
Heated  to  1150  deg.  C.  for  two  hours  and  cooled  in  air.     (Boynton.) 


in  annealing,  namely,  the  production  of  a  very  fine  structure  possessing  high  strength 
and  elastic  limit  with  fair  ductility,  in  other  words  toughness  and  high  resistance  to 
wear  and  to  shock.  The  change  of  structure  taking  place  on  heating  hardened  steel 
close  to  the  lower  limit  of  its  critical  range  will  be  considered  at  some  length  in 
Lesson  XIV.  It  will  suffice  to  note  here  that  the  metal  passes  from  a  fine  martensitic 
or  troostitic  condition  (the  ordinary  condition  of  well-hardened  steel)  to  an  equally 
fine  sorbitic  condition,  possessing  in  a  high  degree  the  physical  properties  desired. 
The  first  heating  is  sometimes  called  "grain  refining"  treatment  and  the  second 
"toughening"  treatment. 

The  quenching  of  a  piece  of  steel  from  above  its  critical  range,  while  simple  enough 
in  the  case  of  very  mild  steel,  presents  increasing  difficulties  as  the  carbon  increases. 
It  should  be  conducted  with  care  and  intelligence  and  only  by  experts.  Steel  con- 


LESSON   XII  —  THE   ANNEALING   OF   STEEL 


9 


taining  very  little  carbon,  say  not  over  0.15  per  cent,  may  be  quenched  in  water, 
others  should  be  quenched  in  oil.  The  Committee  on  Heat  Treatment  of  the  Amer- 
ican Society  for  Testing  Materials  recommends,  in  order  to  lessen  the  danger  of 


Fig.  11.— Steel.  Carbon  0.10  per  cent.  Magnified  100 
diameters.  Heated  to  950  deg.  C.  and  quenched  in 
water.  (Boylston.) 


Fig.  12.  —  Steel.  Carbon  0.20  per  cent.  Magnified 
100  diameters.  Heated  to  850  deg.  C.  and  quenched 
in  oil.  (Boylston.) 

cracking,  that  the  object  be  removed  from  the  oil  or  water  bath  before  its  tempera- 
ture has  fallen  below  160  deg.  C.,  or  in  any  event  below  100  deg.,  and  that  the  second 
treatment  be  applied  within  a  few  hours  after  the  quenching,  preferably  without  ever 


10  LESSON  XII  — THE  ANNEALING   OF  STEEL 

allowing  the  piece  to  cool  below  100  cleg,  and  certainly  not  below  20  deg.  The  final 
properties  of  the  steel  will  depend  upon  the  temperature  of  the  second  heating;  the 
higher  that  temperature  the  softer  and  more  ductile  will  it  be,  but  also  the  less  strong 
and  elastic.  For  great  strength,  high  elastic  limit,  and  little  ductility  reheating  to 
500  deg.  should  be  applied,  while  for  great  ductility,  at  the  sacrifice  of  considerable 
strength,  the  reheating  should  be  carried  to  700  or  725  deg.  For  intermediate  tensile 
strength,  elastic  limit,  and  ductility  such  as  are  desired  in  the  majority  of  cases,  the 
temperature  of  the  second  treatment  should  be  between  600  and  650  deg.  C.  While 
from  purely  theoretical  considerations  it  might  be  argued  that  the  rate  of  cooling 
from  this  second  treatment  is  immaterial,  there  is  little  doubt  but  that  the  strength 
of  the  steel  increases  somewhat  and  its  ductility  decreases  with  the  rapidity  of  cool- 
ing. This  cooling  may  be  performed  in  the  furnace,  in  air,  in  oil,  or  in  water. 


Fig.  13.  —  Steel.  Carbon  0.50  per  cent.  Magnified 
500  diameters.  Heated  to  850  deg.  C.,  quenched  in 
water,  reheated  to  600  deg.,  and  cooled  in  air.  (W.  H. 
Knight  in  the  author's  laboratory.) 

The  double  annealing  treatment  described  in  the  foregoing  paragraphs  was  first 
suggested  by  Wallerant  of  the  Creusot  Steel  Works,  France.  It  was  also  described 
by  Andr6  Le  Chatelier  and  adopted  by  the  French  navy.  Its  use  is  now  general 
when  high  physical  requirements  are  to  be  met. 

In  Figure  13  is  shown  the  structure  of  steel,  containing  some  0.50  per  cent  carbon, 
after  double  annealing.  The  fineness  of  the  structure  should  be  noted  as  well  as  the 
lack  of  laminations  and  the  absence  of  free  ferrite.  This  steel  is  composed  wholly 
of  finely  divided  sorbite. 

Annealing  Eutectoid  Steel.  —  WThile  the  mechanism  of  the  structural  changes 
taking  place  on  annealing  steel  has  been  made  clear  in  the  preceding  pages,  it  may 
not  be  without  interest  to  consider  further  and  in  succession  the  annealing  of  eutee- 
toid,  hypo-eutectoid,  and  hyper-eutectoid  steel,  as  these  three  types  of  steels  have 
different  structures  and  their  annealing  involves  different  structural  changes. 


LESSON   XII  — THE   ANNEALING   OF   STEEL 


11 


Slowly  cooled  eutectoid  steel  is  composed  wholly  of  pearlite  which,  upon  being 
heated  through  the  single  critical  point  of  the  metal,  namely  Ac3.2.i,  is  converted  into 
a  solid  solution  (austenite).  The  grains  of  this  austenite  are  very  fine  as  the  steel 
emerges  from  its  range  and  they  are  kept  from  growing  by  preventing  the  steel  from 
reaching  a  higher  temperature.  On  cooling  through  Ar3.2.i  the  metal  again  becomes 
pearlitic  if  it  be  given  time,  as,  for  instance,  in  cooling  in  the  furnace,  while  it  becomes 
sorbitic  if  cooled  more  quickly,  as,  for  instance,  in  air  in  the  case  of  small  objects. 
Should  the  steel  be  quenched  in  water  or  oil  from  the  annealing  temperature  and 
then  reheated  near  but  below  the  point  Acs.2.i,  the  finely  martenso-troostitic  struc- 
ture produced  by  quenching  from  above  the  range  is  converted  into  very  fine  sorbite. 


Fig.  14.  —  Steel.  Eutectoid.  Magnified  412  diameters.  Heated  to  800  deg. 
C.  and  slowly  cooled  in  furnace.  (C.  C.  Buck,  Correspondence  Course 
student.) 

It  has  been  explained  in  Lesson  X  that,  for  like  treatments,  the  structure  of  eutec- 
toid steel  is  finer  than  that  of  either  hypo-eutectoid  or  hyper-eutectoid  steel  and  this 
holds  true  in  the  case  of  annealed  samples,  although  the  difference  may  not  be  notice- 
able when  comparing  the  structure  of  eutectoid  steel  with  that  of  hyper-eutectoid 
steel  containing  but  a  slight  excess  of  free  cementite. 

In  Figure  14  is  shown  the  structure  of  eutectoid  steel  heated  to  800  deg.  C.  and 
slowly  cooled  in  the  furnace.  It  is  made  up  of  well-developed  pearlite.  The  struc- 
ture of  the  same  steel,  quenched  in  oil  at  825  deg.,  reheated  to  650  deg.,  and  cooled 
in  air,  is  exhibited  in  Figure  15.  The  metal  is  now  composed  of  fine  grained  sorbite. 

Annealing  Hypo-Eutectoid  Steel.  —  Slowly  cooled  hypo-eutectoid  steel  is  an  ag- 
gregate of  pearlite  and  free  ferrite.  On  being  heated  through  its  critical  range,  as 
soon  as  the  point  Aci  is  reached,  the  pearlite  is  bodily  converted  into  austenite,  while 
the  ferrite  still  remains  free.  On  further  heating,  however,  it  begins  to  be  absorbed 
by  austenite,  its  absorption  being  completed  as  the  metal  emerges  from  its  Ac3  point. 


12  LESSON  XII  — THE  ANNEALING  OF  STEEL 

Above  Acs  the  steel  is  composed  wholly  of  homogeneous  austenite.  On  cooling 
through  the  critical  range,  unless,  indeed,  the  cooling  be  very  rapid  and  sufficient 
carbon  be  present,  ferrite  is  again  liberated  in  amount  proportional  to  the  slowness 
of  the  cooling  up  to  the  maximum  quantity  consistent  with  the  carbon  content  in 
the  steel.  If  the  cooling  be  very  slow  then,  for  instance  in  the  furnace,  the  totality 
of  the  excess  ferrite  will  be  rejected  and  the  residual  austenite  converted  into  well- 
defined  pearlite  (Figs.  3  and  7),  while  if  the  cooling  be  more  rapid,  for  instance  in 
air  in  the  case  of  small  objects  or  in  oil  with  larger  ones,  a  portion  only  of  the  excess 
ferrite  is  liberated  while  the  residual  austenite  is  converted  into  sorbite  (Figs.  4 
and  9).  The  liberation  of  ferrite  taking  place  during  the  slow  cooling  of  hypo-eutectoid 
steel  coarsens  its  structure  and  is  the  chief  reason  why  annealed  hypo-eutectoid  steel 


Fig.  15.  —  Steel.  Eutectoid.  Magnified  720  diameters. 
Heated  to  825  deg.  C.,  quenched  in  oil,  reheated  to 
650  dog.,  and  cooled  in  air.  (Boylston.) 


cannot  have  as  fine  a  structure  as  annealed  eutectoid  steel.  Howe  further  contends 
that  as  hypo-eutectoid  steel  is  heated  from  Acj  to  Ac3  a  new  crystalline  growth  takes 
place  which  is  the  coarser  the  greater  the  distance  between  AI  and  A3,  that  is,  the  less 
carbon  in  the  steel,  so  that  by  the  time  the  old  structure  has  been  obliterated,  i.e.  at 
Ac3,  a  new  grain  has  formed  which  is  an  additional  reason  why  the  structure  of  hypo- 
eutectoid  steel  cannot  be  refined  to  the  same  extent  as  that  of  eutectoid  steel. 

The  structure  of  hypo-eutectoid  steel  after  double  annealing  has  been  shown  in 
Figure  13.  The  rapid  cooling  through  the  range  prevented  the  liberation  of  ferrite, 
while  the  second  treatment  produced  sorbite,  but  this  sorbite  is  not  as  fine  grained 
as  that  produced  in  eutectoid  steel  by  similar  treatment. 

Annealing  Hyper-Eutectoid  Steel.  —  Slowly  cooled  hyper-eutectoid  steel  is  an 
aggregate  of  pearlite  and  free  cementite.  On  heating  it  through  its  critical  range,  i.e. 
through  its  Aca.2.i.  and  Accm  points,  pearlite  is  converted  into  austenite  at  the  lower 
point  and  this  austenite  absorbs  the  free  cementite  as  the  metal  is  further  heated 


LESSON   XII  — THE   ANNEALING   OF   STEEL 


13 


from  Ac3.2.i  to  Accm.  At  Accm  the  absorption  is  complete  and  the  metal  composed 
entirely  of  austenite.  On  cooling  through  the  range,  if  time  be  given,  as  for  instance 
in  cooling  in  the  furnace,  the  full  proportion  of  free  cementite  is  again  liberated  and 
the  residual  austenite  converted  at  Ar3.2.i  into  clearly  laminated  pearlite  (Fig.  16). 
If  the  cooling  be  more  rapid,  as  for  instance  in  cooling  small  pieces  in  air,  a  portion 
only  of  the  free  cementite  is  set  free,  while  the  residual  austenite  is  converted  into 
sorbite.  This  setting  free  of  cementite,  like  the  liberation  of  ferrite  in  hypo-eutec- 
toid  steel,  coarsens  the  structure.  The  coarsening  influence  of  free  cementite,  how- 
ever, is  far  from  being  as  marked  as  that  of  free  ferrite,  chiefly  because  free  cementite 
is  generally  present  in  much  smaller  proportions.  Steel  containing  as  much  as  1.50 


Fig.  16.  —  Steel.  Carbon  1.43  per  cent.  Magnified  500  diameters. 
Heated  above  critical  range  and  slowly  cooled  in  furnace.  (Boyn- 
ton.) 


per  cent  carbon,  for  instance,  contains  but  11.50  per  cent  of  coarsening  cementite, 
while  steel  with  0.40  carbon  contains  52  per  cent  of  coarsening  ferrite. 

In  Figure  17  is  shown  the  structure  of  hyper-eutectoid  steel  subjected  to  the 
double  annealing  treatment.  This  treatment  prevented  the  separation  of  any  free 
cementite  and  resulted  in  the  production  of  fine  grained  sorbite. 

Annealing  Steel  Castings.  —  It  has  been  mentioned  that  the  very  coarse  struc- 
ture of  steel  castings,  called  "ingotism"  by  Howe,  was  not  as  readily  refined  as  the 
structure  of  steel  forgings,  its  satisfactory  annealing  often  necessitating  heating  to 
temperatures  considerably  higher  than  the  critical  range.  Notwithstanding  their 
greater  resistance  to  the  annealing  treatment,  successfully  annealed  castings  may 
possess  physical  properties  fairly  equal  to  those  of  forgings.  In  Figure  18  is  shown 
the  structure  of  cast  steel  containing  some  0.30  per  cent  of  carbon  and  properly  an- 
nealed. The  presence  of  a  relatively  small  amount  of  free  ferrite  will  be  noted.  When 
highly  magnified  the  carbon-holding  constituent  should  have  a  sorbito-pearlitic  ap- 
pearance. 


14 


LESSON   XII  — THE   ANNEALING   OF   STEEL 


Spheroidizing  of  Pearlite-Cementite.  —  On  slow  cooling  through  the  critical  range, 
austenite  of  eutectoid  composition  is  converted  into  pearlite  made  up  of  distinct 


Fig.  17.  —  Steel.  Carbon  1.25  percent.  Magnified  670  diameters.  Heated 
to  800  deg.  C.  and  quenched  in  oil,  reheated  to  600  deg.  and  air  cooled. 
(C.  C.  Buck,  Correspondence  Course  student.) 


<•#*&::.' 


Fig.  18.  —  Steel.  Cast.  Carbon  0.30  per  cent. 
Magnified  100  diameters.  Annealed.  (W.  J. 
Burger,  Correspondence  Course  student.) 

parallel  plates  or  lamellae  alternately  of  ferrite  and  cementite.  This  condition  of  the 
cementite  of  pearlite,  however,  is  not  final,  it  is  not  structurally  stable,  if  it  can  be 
thus  expressed,  for  if  the  steel  be  kept  for  a  sufficiently  long  time  at  a  temperature 


LESSON  XII  — THE  ANNEALING  OF  STEEL  15 

but  slightly  below  the  range,  preferably  between  600  and  700  deg.  C.,  the  cementite 
shows  a  marked  tendency  to  form  rounded  particles  or,  as  it  has  been  well  said,  to 
"spheroidize."  This  phenomenon  is  shown  in  Figure  19  in  the  case  of  1.24  per  cent 
carbon  steel.  The  cementite  now  occurs  as  small  irregular  grains  embedded  in 
ferrite.  This  variety  of  pearlite,  if  we  can  still  speak  of  it  as  pearlite,  is  sometimes 
called  "granular"  pearlite.  Some  writers  state  that  the  essential  factor  in  this 
spheroidizing  process  is  extremely  slow  cooling  between  700  and  600  deg.  Since 
the  point  Ari  generally  occurs  a  little  below  700  deg.  it  may  well  be  asked  whether 
(1)  the  spheroidizing  of  pearlite  is  due  to  excessively  slow  cooling  through  that 
point  or  whether  (2)  lamellar  pearlite  must  first  be  formed  by  moderately  slow  cool- 
ing through  Ari,  being  afterwards  converted  into  Spheroidized-  pearlite  below  An. 
The  evidences  at  hand  are  not  conclusive.  Spheroidized  pearlite  is  softer,  less 
tenacious,  and  more  ductile  than  lamellar  pearlite.  The  author  has  heard  of  this 


Fig.  19. — Steel.  Carbon  1.24  percent.  Mag- 
nified 1000  diameters.  Spheroidized  cemen- 
tite. (Osmond.) 

spheroidizing  treatment  having  been  applied  to  high  carbon  steel  in  order  to  soften 
it  so  as  to  facilitate  its  machining,  it  being  afterwards  reheated  above  its  range  and 
made  pearlitic,  sorbitic,  or  martensitic  according  to  requirements. 

Varieties  of  Pearlite.  —  From  the  foregoing  it  will  be  evident  that  several  varieties 
of  pearlite  are  to  be  considered  and  that  the  physical  properties  of  steel  will  depend 
greatly  upon  the  character  of  the  pearlite  it  contains.  Arnold  considers  four  varieties 
of  pearlite  which  are  well  illustrated  in  Figure  20.  His  first  phase,  which  he  calls 
"sorbitic"  pearlite,  is  generally  called  sorbite  by  other  writers.  The  character  and 
physical  properties  of  sorbite  have  been  described,  as  well  as  some  of  the  conditions 
necessary  to  its  formation.  His  second  and  third  phases,  to  which  he  gives  the  names 
respectively  of  "normal"  and  "laminated"  pearlite,  are  both  true  pearlite,  the  thicker 
lamella  of  the  latter  being  due  to  a  slower  cooling  through  the  critical  range.  His 
fourth  phase  is  pearlite  in  the  process  of  spheroidizing. l 

Graphitizing  of  Cementite.  —  It  will  be  explained  in  another  lesson  that  the  car- 
bide FeaC  (cementite)  is  not  the  most  stable  form  that  can  be  assumed  by  carbon 


16 


LESSON   XII  — THE   ANNEALING   OF   STEEL 


Showing  the  Properties  of  Pearlite  and  its  Decomposition  Product. 
Fe3C  represented  Black. 


Mechanical  Properties  of 

Mass. 


Microstructure. 


Segregation  Stages. 


Maximum  tensile  stress 
about  70  tons  per  square 
inch.  Elongation  on  2 
inches =about  10  per  cent. 


1ST  PHASE. 
"Sorbitic"    !  pearlite    with 


emulsified    Fe3C. 
dark  on  etching. 


Verv 


Maximum  tensile  stress 
about  55  tons  per  square 
inch..  Elongation  on  2 
inches=about  15  per  cent. 


Maximum  tensile  stress 
about  35  tons  per  square 
inch.  Elongation  on  2 
inches=about  5  per  cent 


2ND  PHASE. 

Normal  pearlite  with  semi- 
segregated  FejC.  Dark 
on  etching. 


3RD  PHASE. 

Laminated  pearlite  with 
completely  segregated 
Fe3C.  Exhibiting  a  play 
of  gorgeous  colours  when 
lightly  etched. 


Maximum  tensile  stress 
about  30  tons  per  square 
inch. 


4TH  PHASE. 

Laminated  pearlite  passing 
into  massive  Fe3C  and 
ferrite. 


NOTE. — It  is  important  to  remember  that  in  a  single  section  of  steel  two  or  even  all 
three  phases  of  pearlite  may  be  observed  in  juxtaposition  gradually  merging  into  each 
other. 


Fig.  20.  —  (Arnold). 


LESSON   XII  — THE   ANNEALING   OF   STEEL  17 

when  alloyed  with  iron.    It  will  he  shown  that  cementite  tends  to  break  up  into  iron 
and  graphite  according  to  the  reaction 

Fe3C      =    3Fe    +       C 

I  I  I 

cementite      ferrite      graphite 

and  that  the  graphite  form  is  the  final  stable  condition  of  carbon.  This  graphitizing 
tendency  of  cementite  remains  latent  unless  the  conditions  be  favorable  to  its  activity. 
These  conditions  are  (1)  long  exposure  to  a  temperature  exceeding  the  critical  range 
and  slow  cooling,1  (2)  the  presence  of  much  carbon,  and  (3)  the  presence  of  silicon  or 
of  some  other  elements  exerting  a  similar  influence.  It  will  be~shown  that  this  ten- 
dency of  cementite  to  be  converted  into  graphite  and  iron  is  responsible  for  the  pro- 
duction of  so-called  malleable  castings  and,  probably,  also  for  the  production  of  gray 
cast  iron.  In  the  case  of  steel,  because  of  the  relatively  small  amount  of  carbon 
present  (not  exceeding  1.75  or  at  the  most  2  per  cent),  the  graphitizing  tendency  is 
slight.  Long  exposure  of  hyper-eutectoid  steel,  especially  if  it  contains  more  than 
one  per  cent  carbon,  however,  to  a  temperature  exceeding  its  critical  range,  is  always 
likely  to  produce  a  small  amount  at  least  of  graphitic  carbon  greatly  impairing 
thereby,  if  not  ruining,  the  metal.  An  instance  of  graphite  formation  in  high  carbon 
steel  is  shown  in  Figures  21  and  22.  There  is  little  doubt  but  the  free  cementite  pres- 
ent in  hyper-eutectoid  steel,  and  formed  as  the  steel  cools  from  its  Arcm  to  its  Ar3.2.i. 
point,  is  more  readily  converted  into  graphite  than  the  cementite  included  in  the 
pearlite.  Once  the  graphitizing  is  started,  however,  it  may  be  carried  to  completion 
and  include  the  whole  of  the  cementite  present.  This  indeed  is  what  happens  in 
certain  grades  of  malleable  cast  iron  and  of  gray  cast  iron  which  contain  practically 
the  totality  of  their  carbon  in  the  form  of  graphite.  The  presence  of  some  free  cemen- 
tite appears  to  be  necessary  to  start  the  graphitizing,  which  would  explain  why  it 
does  not  take  place  in  hypo-eutectoid  steel.  The  author  at  least  never  had  a  case 
of  graphite  formation  in  such  steels  brought  to  his  attention  nor  was  he  ever  able  to 
produce  graphite  in  hypo-eutectoid  steel. 

Burnt  Steel.  —  When  high  carbon  steel  is  heated  to  a  temperature  approaching 
its  melting-point,  it  becomes  extremely  red  short  as  well  as  cold  short,  its  fracture 
becomes  very  coarse  and  shiny  and  these  defects  cannot  be  cured  short  of  remelting 
the  metal.  The  steel  in  such  condition  is  said  to  be  burnt.  These  results  are  ap- 
parently brought  about  by  the  evolution  of  gases  under  the  influence  of  a  high  tem- 
perature, chiefly  carbon  monoxide,  resulting  from  atmospheric  oxygen  finding  its  way 
through  the  pores  of  the  metal  and  combining  with  some  of  the  carbon,  although 
according  to  Howe  other  occluded  gases,  such  as  hydrogen  and  nitrogen,  may  also 
contribute.  These  gases  force  the  crystalline  grains  apart,  destroying  their  cohesion, 
hence  the  brittleness  of  the  metal.  Oxidized  membranes  are  also  frequently  found 
surrounding  some  of  the  grains,  their  presence  readily  explaining  the  impossibility 
of  restoring  burnt  steel  by  forging  since  they  would  prevent  the  welding  of  adjacent 
grains.  Instances  of  the  structure  of  burnt  steel  are  shown  in  Figures  23  and  24.  Howe 
defines  burning  as  being  "a  mechanical  separation  of  the  grains  on  extreme  overheat- 
ing." Some  writers  have  argued,  apparently  on  good  ground,  that  burning  will  not 
take  place  unless  the  steel  has  been  heated  to  so  high  a  temperature  that  it  has  actu- 
ally begun  to  melt,  the  explanation  being  perfectly  consistent  with  the  well-known 

1  One  or  two  instances  have  been  cited  of  graphite  having  been  formed  below  the  critical  range. 


18 


LESSON   XII  — THE   ANNEALING   OF   STEEL 


S^mmM^^  £*4f  wl 


':iit 

p*se 


Fig.  21.  —  Stool.  Carbon  1.25  per  cent.  Magnified  150  diameters.  An- 
nealed five  hours  at  830  dog.  C.  (C.  C.  Buck,  Correspondence  Course 
student.) 


f8&W*^lPijli 


Fig.  22.  —  Steel.  Carbon  1.25  per  cent  Magnified  670  diameters.  An- 
nealed five  hours  at  830  dog.  C.  'C.  C.  Buck,  Correspondence  Course 
student.) 


LESSON  XII  — THE  ANNEALING  OF  STEEL  19 

fact  that  high  carbon  steel  burns  much  more  readily  than  low  carbon  steel.  To  make 
the  matter  clear  let  us  consider  the  diagram  of  Figure  25  in  which  the  solidification 
period  of  steel  is  shown  as  influenced  by  its  carbon  content.  This  diagram  will  be 
discussed  at  greater  length  in  another  lesson.  Let  us  for  the  present  note  (1)  that 
as  the  carbon  increases  from  0  to  2.0  per  cent  the  solidification  of  the  steel  is  lowered 
from  A  to  B,  that  is  from  1500  deg.  C.  to  1325  cleg.,  (2)  that  while  carbonless  iron 
solidifies  at  a  constant  temperature,  namely  1500  deg.,  as  the  carbon  increases,  the 
range  of  temperature  covered  by  the  solidification  period  increases  likewise,  extend- 
ing from  B  to  C  with  2  per  cent  carbon,  that  is  from  1325  to  1130  deg.  C.  ABC  then 
represents  the  solidification  zone  of  steels  of  increasing  carboii  content  and  the 
heating  of  the  metal  to  any  point  within  this  zone,  when  it  is  partly  melted,  will 
cause  it  to  burn.  It  follows  from  this  that  carbonless  iron  and  very  low  carbon  steel 


4 


Fig.  23.  —  Burnt  steel.     Carbon  1.24  per  cent.  Fig.  24.  —  Burnt  steel.     Magnified  30 

Magnified  20  diameters.    Quenched  at  a  white  diameters.    (Stead.) 

heat.    Unetched.    (Osmond.) 

can  be  heated  to  a  very  high  temperature  without  burning,  while  the  danger  of  burn- 
ing increases  with  the  carbon.  With  0.50  per  cent  carbon,  for  instance,  the  burning 
zone  extends  from  1400  to  1450  deg.,  with  1.0  per  cent  it  extends  from  1310  to  1400 
deg.,  with  1.50  per  cent  carbon  from  1210  to  1360  deg.  In  short,  as  the  carbon  in- 
creases the  steel  burns  more  readily  (1)  because  its  melting-point  is  lowered  and  (2) 
because  its  solidification  zone,  which  is  also  its  burning  zone,  is  widened.  According 
to  the  theory  it  should  not  be  possible  to  burn  carbonless  iron,  and  indeed  the  author 
docs  not  know  that  the  claim  has  ever  been  made  that  carbonless  iron  could  be  burnt. 
If  ABC  represents  a  burning  zone  into  which  steel  cannot  be  brought  without 
having  its  useful  qualities  destroyed,  we  naturally  ask  why  all  steels  are  not  so  injured 
seeing  that  they  must  pass  at  least  once  through  this  zone  in  cooling  from  the  molten 
condition.  The  reason  why  steel  does  not  burn  on  solidifying  and  further  cooling  is 
explained  by  Howe  on  the  ground  that  while  steel  ingots  or  other  castings  solidify, 
much  hydrogen  is  given  out  which  may  mechanically  restrain  the  oxygen  from  enter- 
ing and  also  counteract  it,  preventing  thereby  the  evolution  of  CO  from  within  and 
the  formation  of  oxidized  films,  the  chief  causes  of  burning.  It  is  also  possible,  Howe 
says,  that  the  greater  kneading  which  an  ingot  undergoes  cures  burning,  while  the 


20 


LESSON   XII  — THE  ANNEALING  OF  STEEL 


slight  kneading  possible  in  reworking  a  steel  bar  does  not.  This,  however,  would  not 
explain  why  steel  castings,  which  undergo  no  work  at  all,  are  not  burnt,  unless  it  is 
because  they  are  generally  protected  from  atmospheric  oxidation  by  their  molds 
(Howe). 

Burning  should  not  be  confounded  with  overheating.  Overheated  steel  has  a 
very  coarse  structure  and  fracture,  but  it  can  be  restored  by  heat  treatment  alone,  or 
at  least  by  heating  and  forging,  while  burnt  steel  is  incurable.  Overheating  results 
from  heating  close  to  but  below  AC  (Fig.  25),  generally  for  a  considerable  length  of 
time,  while,  as  explained,  the  temperature  in  burning  is  carried  above  AC. 


/5OO 


<0 


Carbon    per    cenf 

Fig.  25.  —  Diagram  depicting  the  burning  temperature  range. 

Important  results  recently  obtained  by  Gutowsky  would  place  the  end  of  the 
solidification  of  various  carbon  steels  as  indicated  by  the  dotted  line  in  Figure  25. 
If  these  are  the  correct  temperatures  at  which  solidification  is  complete,  it  follows 
that  the  burning  zone  is  wider  than  was  generally  believed  before  the  publication  of 
these  results. 

Crystalline  Growth  of  Austenite  Above  the  Critical  Range.  —  Above  its  critical 
range  steel  is  composed  of  polyhedric  crystalline  grains  of  austenite,  which  are  made 
up  of  small  crystals  (probably  octahedra)  similarly  oriented  in  the  same  grain  but 
whose  orientation  changes  from  one  grain  to  the  next.  Indeed  it  is  this  lack  of  uni- 
formity of  the  orientation  of  the  crystalline  matter  building  up  the  grains  that  gives 
existence  to  these  grains,  for  if  all  the  small  crystals  of  which  they  are  composed  were 
similarly  oriented,  clearly  there  would  be  but  a  single  allotrimorphic  crystal  or  grain. 


LESSON  XII  — THE  ANNEALING  OF  STEEL  21 

If  steel  be  maintained  for  a  long  time  above  its  critical  range,  the  austenite  grains  of 
which  it  is  composed  show  a  tendency  to  grow  in  size  through  adjoining  grains  assu- 
ming like  crystalline  orientation  and,  therefore,  merging  into  a  single  and  correspond- 
ingly larger  grain.  This  growth  increases  with  the  temperature  and  with  the  duration 
of  the  treatment.  Given  a  sufficiently  long  time  and  sufficiently  high  temperature, 
but  one  grain  must  be  formed.  This,  as  already  seen  in  Lesson  X,  is  actually  what 
takes  place  in  meteorites  during  the  cooling  of  which  the  prevailing  conditions  are 
such  as  to  produce  this  uniformity  of  orientation.  It  follows  from  the  above  con- 
siderations that  on  annealing,  if  the  metal  be  kept  a  long  time  above  its  critical  range, 
even  but  slightly  above  it,  a  coarser  austenite  will  be  formed  which  in  turn  implies, 
after  slow  cooling,  a  coarser  pearlitic  or  sorbitic  structure.  In  hypo-eutectoid  steel 
the  grains  of  austenite  will  expel  some  free  ferrite,  and  in  hyper-eutectoid  steel  some 
free  cementite,  before  being  converted  into  pearlite,  but  the  final  pearlite  grains  will 
nevertheless  increase  in  size  with  the  size  of  the  original  austenite  grains.  The 
structure  of  steel  containing  0.50  per  cent  carbon  kept  two  hours  at  1150  deg.  C. 
and  cooled  in  air  has  been  shown  in  Figure  10.  The  very  large  sorbitic  grains 
formed  prove  the  existence  above  the  range  of  equally  large,  or  even  larger,  austenitic 
grains.  The  cooling  through  the  range  was  so  rapid  that  but  a  small  amount  of 
free  ferrite  was  separated,  the  sorbite  grains,  therefore,  representing  nearly  the  exact 
size  of  the  austenite  grains. 

In  Figure  26  an  attempt  has  been  made  to  depict  this  relation  between  the 
austenitic  structure  above  the  range  and  the  corresponding  pearlitic  or  sorbitic  struc- 
ture below  the  range.  The  steel  considered  is  supposed  to  be  hypo-eutectoid  and  to 
contain,  after  slow  cooling,  a  large  proportion  of  free  ferrite.  A  is  intended  to  rep- 
resent a  piece  of  this  steel  made  up  of  nine  relatively  small  austenitic  grains  formed 
on  short  exposure  above  the  range.  On  cooling  through  the  range  ferrite  is  liberated 
and  the  residual  austenite  grains  converted  into  as  many  pearlite  grains  as  shown  in 
A'.  The  small  squares  of  the  matrix  surrounding  the  pearlite  grains  represent  as 
many  small  ferrite  grains.  After  a  longer  exposure,  possibly  at  a  higher  temperature, 
the  steel  will  be  made  up  of  larger  austenite  grains,  say  of  four  grains  as  shown  in  B, 
and  these,  on  slow  cooling  through  the  range,  will  be  converted  after  rejection  of  fer- 
rite into  four  pearlite  grains  as  indicated  in  B'.  Theoretically,  at  least,  we  may  assume 
that  the  temperature  above  the  range  may  be  so  high  and  the  exposure  at  that  tem- 
perature so  long  that  but  a  single  austenite  grain  is  formed,  the  entire  mass  having 
assumed  a  uniform  crystalline  orientation  as  shown  in  C.  On  slow  cooling  a  single 
pearlite  grain  would  then  be  formed  surrounded  by  free  ferrite  in  the  form  of  small 
grains  as  depicted  in  C'.  An  exceedingly  slow  cooling  through  and  below  the  range 
would  have  a  tendency  to  cause  the  free  ferrite  to  crystallize  into  larger  grains  and 
eventually  to  form  but  a  single  grain  as  indicated  in  C".1  Finally,  as  will  be  later 
explained,  on  very  long  exposure  to  a  high  temperature  the  cementite  should,  theo- 
retically at  least,  be  converted  into  as  many  graphite  particles  as  there  are  austenite 
grains  and,  therefore,  into  a  single  graphite  particle  in  case  there  is  but  a  single  grain 
of  austenite,  the  steel  then  consisting,  after  slow  cooling,  of  a  kernel  of  graphite  sur- 
rounded by  uniformly  oriented  ferrite  as  shown  in  C'".  As  already  explained, 
however,  this  breaking  up  of  cementite  into  graphite  and  ferrite  does  not  take  place 
unless  a  considerable  amount  of  carbon  is  present,  namely  over  1  per  cent. 

1  Such  slow  cooling,  as  previously  explained,  would  also  have  a  tendency  to  cause  the  spheroi- 
dizing  of  cementite. 


22 


LESSON   XII  — THE   ANNEALING   OF   STEEL 


LESSON   XII  — THE   ANNEALING   OF   STEEL  23 

The  conditions  depicted  in  A',  B' ,  C',  C",  and  C'"  are  conditions  of  equilibrium, 
according  to  the  phase  rule,  since  but  two  phases  are  present.  It  is  now  believed, 
however,  that  A',  B',  C',  and  C"  represent  metastable  equilibrium  while  D,  only, 
represents  stable  equilibrium.  The  phase  rule  will  be  considered  in  another  lesson 
when  these  remarks  will  be  made  clear. 

Crystalline  Growth  of  Ferrite  Below  the  Critical  Range.  —  Stead  in  1898  showed 
that  strained  ferrite,  when  heated  close  to  but  below  the  critical  range  of  the  metal, 
undergoes  a  marked  crystalline  growth  caused  by  adjoining  ferrite  grains  assuming 
the  same  crystalline  orientation  and  therefore  merging  into  a  larger  grain.  It  seemed 
evident  from  Stead's  experiments  that  unless  the  ferrite  be  strained  (through  cold 
working  or  otherwise)  it  will  not  grow  on  annealing,  and  also  -Htafc  the  presence  of  a 
relatively  small  amount  of  carbon  (some  0.15  per  cent  or  more)  effectively  prevents 
the  growth  of  the  ferrite  grains,  apparently  because  of  the  pearlite  particles  standing 


Fig.  27.  —  Steel.  Carbon  0.05  per  cent.  .Magnified  6  diameters.  Subjected  to 
Brinell  ball  test  under  pressure  of  6000  kilograms  and  heated  to  650  deg.  C.  for 
seven  hours.  Vertical  section  through  bottom  of  spherical  depression.  (J.  O. 
Connolly  in  the  author's  laboratory.) 

in  their  way  and  preventing  their  merging.  The  author  recently  conducted  in  his 
laboratory  some  experiments,  the  results  of  which  throw  additional  light  upon  this 
crystalline  growth.1 

The  following  extracts  from  a  paper  by  the  author  to  be  presented  at  the  sixth 
Congress  of  the  International  Association  for  Testing  Materials  describe  briefly  some 
of  the  most  significant  results  obtained. 

In  Figure  27  is  shown  the  slightly  magnified  structure  of  a  steel  containing  0.05 
per  cent  carbon  which  had  been  subjected  to  the  Brinell  ball  test 2  under  a  pressure  of 
6000  kilograms  and  then  annealed  at  650  dog.  C.  for  seven  hours.  The  section  shown 
is  a  vertical  one  passing  through  the  bottom  of  the  spherical  depression  made  by  the 

1  These  experiments  were  made  by  J.  O.  Connolly,  at  the  time  a  research  student  at  Harvard 
University,  now  with  the  American  Steel  and  Wire  Co. 

2  In  using  the  Brinell  ball  test  as  a  suitable  means  of  straining  ferrite  in  order  to  study  its 
growth  on  annealing  the  author  followed  Charpy. 


24  LESSON  XII  — THE  ANNEALING  OF  STEEL 

10mm.  ball  used.  It  will  be  obvious  that  the  strain  was  most  severe  at  A,  that  is 
at  the  very  bottom  of  the  depression,  and  that  it  decreased  gradually  in  intensity 
from  A  to  D.  The  following  features  of  the  structure  should  be  noted  as  having 
special  significance:  (1)  At  D  where  the  metal  was  but  slightly  if  at  all  strained  no 
crystalline  growth  occurred,  (2)  at  C  where  the  strain  must  have  been  more  severe  a 
sudden  growth  of  maximum  intensity  has  taken  place,  (3)  from  C  to  B  as  the  severity 
of  the  strain  increases  the  crystalline  growth  shows  a  gradual  decrease,  and  (4)  from 
B  to  A,  that  is,  with  further  increase  of  the  intensity  of  the  strain,  no  crystalline 
growth  has  taken  place.  These  observations  point  to  the  conclusion  that  ferrite 
grains  will  not  grow  on  annealing  below  the  critical  range  unless  they  have  been 
subjected  to  a  certain  stress  creating  a  certain  strain,  and  that  they  will  not  grow  if 


Fig.  28.  —  Steel.  Carbon  0.05  per  cent.  Magnified  6 
diameters.  Subjected  to  Brinell  ball  test  under 
pressure  of  3000  kilograms  and  heated  to  650  deg.  C. 
for  seven  hours.  Vertical  section  through  bottom  of 
spherical  depression.  (J.  O.  Connolly  in  the  author's 
laboratory.) 

that  stress,  and  therefore  the  resulting  strain,  has  been  exceeded.  In  other  words, 
they  point  to  the  existence  of  a  critical  strain  producing  growth,  strains  of  greater 
or  less  magnitude  being  ineffective.  The  narrow  region  occupied  by  the  critically 
strained  metal  should  also  be  noted  as  well  as  the  very  sharp  line  of  demarcation  be- 
tween the  critically  strained  and  the  under-strained  metal.  The  separation  of  the 
critically  strained  metal  from  the  over-strained  is  not  so  sharp. 

Similar  experiments  were  repeated  many  times  and  like  results  always  obtained. 

A  piece  of  the  same  steel  was  subjected  to  exactly  the  same  treatment,  except  that 
the  ball  pressure  applied  was  3000  kilograms  or  half  the  pressure  applied  to  the 
previous  one.  The  crystalline  growth  resulting  from  the  annealing  of  this  sample  is 
shown  in  Figure  28.  Because  of  the  smaller  stress  applied,  the  critically  strained  por- 
tion of  the  metal  is  nearer  the  depression.  This  would  naturally  be  expected.  Here, 
as  in  the  previous  case,  we  have  three  distinct  regions:  (1)  the  metal  surrounding  the 
depression  and  extending  to  a  certain  distance  which  was  too  severely  strained  to 


LESSON   XII  — THE   ANNEALING   OF   STEEL  25 

grow,  (2)  the  critically  strained  metal  in  the  form  of  a  spherical  shell,  and  (3)  the 
rest  of  the  metal  unstrained  or  too  feebly  strained  for  the  growth  to  take  place.  Fig- 
ure 29  is  a  section  through  a  similar  sample,  the  specimen  having  been  ground  level 
with  the  bottom  of  the  depression.  The  occurrence  in  this  section  of  a  ring  showing 
crystalline  growth  will  be  readily  understood. 

In  Figure  30  is  shown  the  structure  of  a  bar  of  the  same  steel  which  after  having 
been  completely  bent  was  subjected  to  annealing  (seven  hours  at  650  deg.  C.).  A 
piece  of  the  bent  portion  of  this  bar  was  then  cut  and  a  longitudinal  section  through 
its  center  prepared  for  microscopical  examination.  It  will  be  obvious  that  the  upper 
part  of  the  bent  portion  of  the  bar  was  subjected  to  severe  tension  and  the  under 
part  to  severe  compression.  Somewhere  between  the  upper— and  lower  parts  a 


Fig.  29.  —  Steel.  Carbon  0.05  per  cent.  Magnified 
6  diameters.  Subjected  to  Brinell  ball  test  under 
pressure  of  3000  kilograms  and  heated  to  650  deg. 
C.  for  seven  hours.  Horizontal  section  through 
bottom  of  spherical  depression.  (J.  O.  Connolly  in 
the  author's  laboratory.) 

neutral  plane  existed  which  was  subjected  neither  to  tension  nor  to  compression,  and 
in  the  vicinity  of  this  plane  the  metal  was  but  slightly  strained.  Moving  in  both 
directions  from  this  neutral  plane,  the  metal  becomes  gradually  more  severely  strained. 
Figure  30  shows  that  (1)  in  the  center  of  the  bar  no  growth  took  place,  the  metal 
being  here  under-strained,  (2)  as  soon  as  the  critically  strained  portion  was  reached 
a  very  abrupt  growth  occurred  of  maximum  intensity,  and  (3)  this  growth  decreased 
gradually  as  the  metal  became  more  severely  strained,  being  very  slight  if  existing 
at  all  where  the  strain  was  maximum,  that  is,  near  the  upper  and  under  parts  of  the 
bend.  The  widening  of  the  central  zone,  free  from  growth  as  the  distance  from  the 
bend  increases,  is  also  consistent  with  the  existence  of  a  critical  strain  for  it  is  evident 
that  the  portion  of  unstrained  or  under-strained  metal  increases  with  that  distance. 
Incidentally  this  experiment  shows  that  tension  is  apparently  as  effective  as  com- 


26  LESSON   XII  — THE   ANNEALING   OF   STEEL 

pression  in  producing  crystalline  growth,  there  being  apparently  no  difference  in  the 
size  of  the  ferrite  grains  between  the  upper  and  under  parts  of  the  bar.  The  criti- 
cally strained  portions  occupy  also  nearly  the  same  position  with  regard  to  the  out- 
side surfaces,  that  is,  they  occur  at  the  same  depth.  Their  width  also  appears  to  be 
the  same. 

Bars  of  the  same  steel  were  subjected  to  compression,  shearing,  and  twisting 
stresses  and  the  results  obtained  were  in  every  case  consistent  with  the  existence  of 
a  critical  strain. 

With  a  view  of  securing  some  data  in  regard  to  the  magnitude  of  the  critical  stress 
needed  to  induce  growth  on  annealing,  a  number  of  bars  of  the  same  steel  were  sub- 
jected to  tensile  stresses  of  increasing  intensity,  annealed  at  650  deg.  seven  hours,  and 
in  every  case  a  cross  section  of  the  strained  and  annealed  bar  was  prepared  for  micro- 


Fig.  30.  —  Steel.  Carbon  0.05  per  cent.  Magnified  3  diameters.  Bar  bent 
double  and  heated  to  650  deg.  C.  for  seven  hours.  Longitudinal  section  through 
center  of  bent  portion.  (J.  O.  Connolly  in  the  author's  laboratory.) 


scopical  examination.  The  elastic  limit  or  rather  yield  point  of  the  metal  was  in  the 
vicinity  of  28,000  Ibs.  per  square  inch,  and  its  ultimate  strength  45,600  Ibs.  per  square 
inch. 

In  Figures  31,  32,  33,  and  34  are  shown  the  structures  of  the  bars  subjected  re- 
spectively to  stresses  of  38,000,  40,000,  42,000,  and  44,000  Ibs.  per  square  inch. 

These  tensile  tests  afford  another  conclusive  evidence  of  the  existence  of  a  critical 
strain  and  the  fact  that  a  stress  of  40,000  Ibs.  per  square  inch  produces  a  marked 
growth  while  stresses  but  slightly  inferior  or  superior,  namely,  38,000  and  42,000  Ibs. 
per  square  inch  do  not  induce  growth  is  an  indication  of  the  narrowness  of  the  range 
of  the  critical  stress. 

Brittleness  of  Low  Carbon  Steel.  —  The  crystalline  growths  and  other  structural 
changes  described  in  the  foregoing  pages  lead  naturally  to  the  consideration  of  the 
brittleness  occasionally  exhibited  by  low  carbon  steel.  Since  steel  containing  very 
little  carbon  is  essentially  made  up  of  ferrite,  its  occasional  brittleness  must  be  due  to 
the  occasional  brittleness  of  ferrite,  a  constituent  by  nature  soft  and  ductile.  Stead 
has  indicated  two  kinds  of  brittleness  from  which  ferrite  may,  and  occasionally  does. 


LESSON   XII  — THE   ANNEALING   OF   STEEL 


27 


suffer,  namely,  (1)  "inter-granular"  brittleness  and  (2)  "inter-crystalline"  or  "cleav- 
age" brittleness. 

By  inter-granular  brittleness  is  meant  a  lack  of  cohesion  between  the  ferrite  grains 
leading  to  ready  fracture  under  shock,  the  line  of  fracture  following  the  boundary 


Fig.  31.  —  Tensile  stress,  38,000  Ibs.'per  sq.  in. 


Fig.  33. —Tensile  stress,  42,000  Ibs.  per  sq.  in. 


Fig.  32.  —  Tensile  stress,  40,000  Ibs.  per  sq.  in. 


Fig.  34.  —  Tensile  stress,  44,000  Ibs.  per  sq.  in. 


Steel.    Carbon  0.05  per  cent.    Magnified  6  diameters.    Strained  by  tension  and  heated  to  650  deg. 
C.  for  seven  hours.    Cross  sections  of  strained  bars.     (J.  O.  Connolly  in  the  author's  laboratory.) 

lines  of  the  grains.  Such  brittleness  is  usually  due  to  the  presence  of  impurities  form- 
ing brittle  and  more  or  less  continuous  membranes  surrounding  the  grains.  The 
presence  of  much  phosphorus,  however,  appears  to  produce  inter-granular  brittleness 
without  producing  surrounding  membranes. 


28 


LESSON  XII  — THE  ANNEALING   OF  STEEL 


Inter-crystalline  or  cleavage  brittleness  is  caused  by  the  ferrite  grains  assuming 
nearly  the  same  crystalline  orientation  so  that  the  plane  of  fracture  follows  the  cleav- 
age planes  and  passes  from  grain  to  grain  almost  in  a  straight  line.  The  diagram 
shown  in  Figure  35  will  make  this  clear.  The  cross  lines  represent  the  cleavage 
planes  in  each  grain.  In  B  the  metal  is  made  up  of  large  ferrite  grains  but  the  crys- 
talline orientation  of  these  grains  is  so  heterogeneous  that  a  line  of  fracture  cannot 
readily  be  developed  and  pass  from  grain  to  grain,  the  abrupt  change  of  crystalline 
orientation  encountered  at  each  boundary  line  acting  as  an  effective  obstruction  to 
its  advance.  In  A,  on  the  contrary,  while  the  grains  are  smaller  they  have  nearly  the 
same  orientation,  hence  fracture  may  proceed  from  grain  to  grain  with  much  greater 
ease  and  in  a  nearly  straight  line.  Fortunately,  so  uniform  a  crystalline  orientation 


Fig.  35.  —  (Stead.) 

is  not  often  met  with  and  cleavage  brittleness  is  a  rather  rare  occurrence.    It  can  be 
cured  by  reheating  the  steel  to  900  deg.  or  higher. 

Stead  also  noticed  that  low  carbon  steel  plates  rolled  below  the  critical  range  and, 
therefore,  strained,  and  annealed  likewise  below  the  range,  often  exhibit  a  tendency 
to  break  in  three  directions,  namely,  at  45  deg.  to  the  direction  of  rolling  and  at  right 
angles  with  the  surface  of  the  plates,  that  is,  in  the  directions  of  the  three  cleavage 
planes  of  a  cube  having  four  faces  at  45  deg.  to  the  edges,  and  two  faces  parallel 
to  the  surface  of  the  plates.  This  he  calls  "rectangular"  brittleness.  "We  are  led 
from  this  to  conclude,"  Stead  writes,  "that,  just  as  light  impresses  a  latent  image  on 
a  bromide  photographic  plate  which  cannot  be  seen  but  is  developed  and  made  mani- 
fest by  the  action  of  certain  chemical  agencies,  so  the  rolling  appears  to  impress  a 
latent  disposition  in  the  steel  to  crystallize  in  certain  fixed  positions,  and  annealing 
develops  it  afterwards."  The  brittleness  here  referred  to  is  undoubtedly  caused  by 
the  crystalline  growth  of  strained  ferrite  when  annealed  below  its  critical  range  as 
fully  explained  in  this  lesson,  the  formation  of  large  ferrite  grains  naturally  causing 
brittleness.  This  kind  of  brittleness  is  sometimes  called  "Stead's  brittleness."  No 
very  satisfactory  explanation  has  so  far  been  offered  to  account  for  this  greater  brit- 


LESSON   XII  — THE   ANNEALING   OF   STEEL  29 

tleness  in  certain  directions.  It  may  be  that  the  large  crystalline  grains  of  ferrite 
produced  have  nearly  the  same  orientation  and  that  they  are  so  oriented  as  to  lead 
to  easy  inter-crystalline  rupture  in  the  directions  indicated. 


Experiments 

The  student  should  procure  samples  of  hypo-  and  hyper-eutectoid  steels,  cast, 
hot  worked,  and  cold  worked.  These  should  be  microscopically  examined  and,  if 
possible,  photographed.  They  should  then  be  annealed,  following  the  instructions 
given  in  this  lesson,  and  cooled  at  various  rates,  to  wit,  in  furnace,  in  air,  and,  in  the 
case  of  low  carbon  steels,  in  oil  or  even  in  water  if  the  steel  does  not  contain  over 
0.15  per  cent  carbon.  The  structure  of  each  sample  should  be  compared  to  the  struc- 
ture of  the  same  steel  before  annealing  and  the  structural  changes  resulting  from  the 
annealing  operation  carefully  noted. 

It  is  advisable  to  subject  at  least  one  sample  of  hypo-eutectoid  steel  and  one  sam- 
ple of  hyper-eutectoid  steel  to  the  double  annealing  treatment,  noting  the  finely 
sorbitic  structure  produced  by  this  operation. 

Samples  of  hyper-eutectoid  steel,  preferably  containing  not  less  than  1.25  per  cent 
carbon,  should  be  subjected  to  (1)  spheroidizing  treatment  and  (2)  graphitizing  treat- 
ment, and  their  structure  carefully  examined. 

A  sample  of  hyper-eutectoid  steel  should  be  heated  into  its  burning  zone  and  the 
resulting  structure  examined. 

A  small  sample  of  steel  containing  not  more  than  0.10  per  cent  carbon  should  be 
filed  very  smooth,  subjected  to  the  ball  test  under  a  pressure  of  some  3000  kilograms, 
and  annealed  to  650  or  700  deg.  for  at  least  five  hours.  A  vertical  section  and  an 
horizontal  section  passing  through  the  bottom  of  the  spherical  depression  should  be 
prepared  for  microscopical  examination  and  the  location  and  magnitude  of  the 
crystalline  growth  noted. 

It  is  desirable  that  all  samples  should  be  photographed,  using  suitable  magnifica- 
tions. 

Examination 

I.  Assuming  a  forged  steel  containing  0.50  per  cent  carbon,  what  annealing  treat- 
ment would  you  recommend  in  order  to  produce:  (1)  great  softness,  (2)  hard- 
ness and  great  strength  but  little  ductility,  and  (3)  a  fair  combination  of 
strength,  elasticity,  and  ductility? 

II.     Assuming  a  steel  containing  0.10  per  cent  carbon,  what  treatment  would  you 
recommend  to  produce  maximum  strength? 

III.  Assuming  a  steel  containing  1.25  per  cent  carbon,  what  treatment  would  you 

recommend  to  produce  a  desirable  combination  of  strength,  elasticity,  and 
ductility  so  that  the  metal  while  tenacious  will  satisfactorily  stand  wear  and 
shocks? 

IV.  Explain  how  steel  can  be  made  sorbitic  by  annealing. 

V.     Explain  the  difference  in  physical  properties  between  sorbitic  and  pearlitic 
steel  containing  the  same  amount  of  carbon. 


30  LESSON  XII— THE  ANNEALING   OF  STEEL 

VI.     Explain  why  the  structure  and  fracture  of  annealed  hypo-eutectoid  steel  can- 
not be  made  as  fine  as  the  structure  and  fracture  of  annealed  eutectoid  steel. 

VII.     Describe  and  explain  the  burning  of  steel. 

VIII.  Explain  the  relation  existing  between  the  dimension  of  austenite  grains  formed 
above  the  critical  range  and  the  pearlite  grains  formed  on  passing  through 
the  range. 


LESSON  XIII 

THE  HARDENING   OF  STEEL 

References  have  already  been  made  in  these  lessons  to  the4nvaluable  property 
possessed  by  iron,  containing  a  sufficient  amount  of  carbon,  of  becoming  extremely 
hard  when  suddenly  cooled  from  a  high  temperature  as,  for  instance,  by  quenching 
in  water  from  a  bright  red  heat.  This  operation  is  known  as  the  hardening  of  steel. 
The  close  relation  existing  between  the  hardening  of  steel  and  its  critical  range,  which 
has  also  been  alluded  to,  provides  the  key  to  the  rationale  of  the  hardening  opera- 
tion. This  operation  consists  of  two  distinct  steps  (1)  heating  to  the  hardening 
temperature  and  (2)  cooling  from  that  temperature. 

Heating  for  Hardening.  —  In  order  to  harden  steel  it  is  necessary  first  to  heat 
it  above  its  critical  range,  because  it  is  in  passing  through  that  range  that  it  acquires 
hardening  power.  Any  attempt  at  hardening  it  by  cooling  it  suddenly  from  a  temper- 
ature inferior  to  its  critical  range  would  result  in  but  a  very  slight,  if  any,  increase  of 
hardness.  It  is  evident,  therefore,  that  to  possess  hardening  power  steel  must  be  in 
the  condition  of  a  solid  solution  since  the  aggregate  of  ferrite  and  cementite  formed  on 
slow  cooling  through  the  critical  range  cannot  be  hardened  by  sudden  cooling.  The 
metal  should  not  be  heated  much  above  the  top  of  its  range,  because  in  so  doing  we 
coarsen  its  structure  as  explained  in  previous  lessons,  while  we  do  not  increase,  mate- 
rially at  least,  its  hardening  power,  and  our  aim  in  hardening  should  be  to  secure 
maximum  hardness  and  finest  possible  structure.  Quenching  from  a  temperature 
greatly  exceeding  the  critical  range,  moreover,  increases  the  danger  of  warping  and 
cracking  the  objects  in  the  quenching  bath.  Nor  should  the  steel  be  heated  to  a 
temperature  much  above  its  critical  range  and  then  cooled  to  that  range  before  quench- 
ing, as  sometimes  recommended,  because  its  structure  is  then  likewise  coarsened  by 
the  heating  and  slow  cooling  preceding  the  quenching.  Clearly  the  rationale  of  the 
hardening  operation  consists  in  heating  the  metal  just  through  its  critical  range, 
thus  conferring  to  it  both  full  hardening  power  and  finest  possible  structure,  and 
then  in  cooling  it  suddenly  as  soon  as  it  emerges  from  its  range,  lest  its  structure  be 
coarsened  by  heating  above  the  range  or  by  prolonged  exposure  at  the  quenching 
temperature.  This  judicious  method  of  conducting  the  hardening  operation  is  some- 
times described  as  "hardening  on  a  rising  temperature."  Let  it  be  borne  in  mind 
that,  since  the  position  and  width  of  the  critical  range  vary  in  different  steels,  the 
most  desirable  quenching  temperature  will  vary  likewise.  Low  and  medium  high 
carbon  steels  should  be  quenched  at  higher  temperatures  than  high  carbon  steel, 
for  in  order  to  acquire  full  hardening  power  they  should  be  heated  past  their  upper 
critical  points,  namely,  Ac3  or  Ac3.2,  as  the  case  may  be. 

Cooling  for  Hardening.  —  To  harden  the  steel  the  metal  should  be  cooled  very 
quickly  from  the  temperatures  mentioned  in  the  above  paragraph  to  atmospheric 
temperature,  generally  by  immersing  it  in  a  medium  capable  of  rapidly  abstract- 
ing heat  from  it.  The  increase  of  hardness  will  lie  the  greater  the  higher  the  carbon 


2  LESSON  XIII  — THE  HARDENING  OF  STEEL 

content,  at  least  up  to  the  eutectoid  point,  and  the  more  rapid  the  cooling,  the 
latter,  in  turn,  depending  upon  the  size  of  the  object  hardened  and  the  nature  of  the 
quenching  bath,  i.e.  its  power  of  abstracting  heat  from  the  cooling  mass.  It  was  long 
thought  that  this  so  to  speak  cooling  power  of  the  bath  depended  chiefly,  if  not 
solely,  upon  its  temperature  at  the  time  of  immersion  and  upon  its  heat  conductivity. 
It  was  believed,  for  instance,  that  mercury  was  a  more  effective  cooling  medium  than 
water,  because  of  its  greater  conductivity  for  heat,  that  cold  water  was  more  effec- 
tive than  tepid  water,  because  of  its  lower  temperature,  etc.  Recent  investigations 
appear  to  show,  however,  that  the  cooling  power  of  a  quenching  bath  is,  within  limits, 
quite  independent  of  its  actual  temperature  and  of  its  heat  conductivity,  and  even  of 
its  specific  heat.  Benedicks  contends  that  it  depends  almost  exclusively  upon  its 
latent  heat  of  volatilization.  Its  temperature,  however,  should  be  low  enough  to 


£? 

^fc  —^^^^  •*^5it  J^^Wf-*JJ   jf  fi^r   i  *•  * 


Fig.  1.  —  Steel.  Carbon  0.45  per  cent.  Mag- 
nified 1000  diameters.  Heated  to  825  deg.  C. 
and  quenched  at  720  deg.  (Osmond.) 


prevent  the  adherence  of  vapor  bubbles  to  the  metal.  In  accordance  with  these 
views  mercury,  in  Benedicks'  opinion,  is  inferior  to  water  while  saline  solutions  are 
not  superior  to  it.  Methyl  alcohol,  on  the  contrary,  is  a  more  effective  cooling  medium 
for  hardening  than  water.  According  to  Le  Chatelier,  also,  mercury  is  less  effective 
than  water  but  in  his  opinion  because  of  its  lower  specific  heat.  Le  Chatelier  be- 
lieves that  the  specific  heat  of  the  liquid  is  the  most  important  factor  influencing  its 
value  as  a  cooling  medium,  its  conductivity  being  of  secondary  importance,  the  loss 
of  heat  taking  place  more  through  circulation  than  through  conductivity.  On  the 
other  hand  Benedicks  contends  that  the  rate  of  flow  has  very  little  influence. 

Structural  Changes  on  Hardening.  —  Bearing  in  mind  the  enormous  differ- 
ence between,  the  properties  of  hardened  steel  and  those  of  the  same  metal  un- 
hardened,  we  should  naturally  expect  to  find  the  structure  of  hardened  steel  likewise 
totally  different  from  that  of  unhardened  steel.  And  so  indeed  it  is  as  shown  in 
Figure  1,  in  the  case  of  hardened  steel  containing  some  0.50  per  cent  carbon.  To 
account  for  this  structure  let  us  remember  that,  initially,  this  steel  consisted  of  an 
aggregate  of  ferrite  and  cementite  which,  upon  being  heated  through  its  critical 


LESSON   XIII  — THE   HARDENING   OF  STEEL  3 

range,  was  converted  into  a  solid  solution  (austenite)  of  carbon  or  carbide  in  gamma 
iron.  This  was  necessary  to  impart  hardening  power.  Had  the  metal  been  allowed 
to  cool  slowly  through  its  critical  range  it  would  have  been  converted  back  into  a 
mixture  of  ferrite  and  cementite.  On  rapid  cooling,  however,  this  transformation 
was  prevented,  at  least  in  part,  the  time  necessary  for  its  completion  having  been 
denied.  A  conclusive  evidence  that  the  transformation  does  not  occur  in  its  entirety  is 
afforded  by  the  absence  of  a  marked  critical  range  on  quick  cooling.  If  the  transforma- 
tion of  the  solid  solution  could  be  effectively  prevented  austenite  should  be  the  constit- 
uent of  hardened  steel.  In  the  commercial  hardening  of  steel,  however,  the  cooling 
is  not  sudden  enough  to  prevent  at  least  a  partial  transformation  of  austenite,  not 
into  ferrite  and  cementite  but  into  a  more  or  less  transitory  form,  marking  the  first 
step  of  that  transformation  and  called  "martensite."  Very  frequently  the  rate  of 


Fig.  2.  —  Steel.  Carbon  1.57  per  cent.  Mag- 
nified 1000  diameters.  Heated  to  1050  deg. 
C.  and  quenched  in  ice-water.  (Osmond.) 

cooling  is  not  sufficiently  rapid  to  prevent  the  martensite  from  further  partial  '-ans- 
formation  into  a  second  transition  constituent  known  as  "troostite."  Martensite  and 
troostite,  then,  are  the  ordinary  constituents  of  commercially  hardened  steel.  It  will 
now  be  profitable  to  consider  at  some  length  the  occurrence,  nature,  and  properties 
of  the  three  constituents  chiefly  concerned  in  dealing  with  hardened  steel,  namely, 
austenite,  martensite,  and  troostite. 

AUSTENITE 

Nature  of  Austenite.1  —  Austenite  is  universally  considered  as  a  solid  solution 
of  carbon  or,  more  probably,  of  the  carbide  Fe3C  in  gamma  iron.2  All  steels  above 
their  critical  range  are  made  up  of  this  solid  solution.  It  follows  that  the  carbon 
content  of  austenite  varies,  like  that  of  steel,  from  a  mere  trace  to  some  1.75  or  2  per 
cent.  It  is  not,  therefore,  a  constituent  of  constant  composition. 

1  This  name  was  suggested  by  Osmond  in  honor  of  the  late  Sir  William  Roberts-Austen.    Aus- 
tenite has  also  been  called  mixed  crystals  and  gamma  iron  and  by  some  writers,  wrongly,  martensite. 

2  Arnold  believes  that  austenite  is  the  carbide  Fe24C  (hardenite)  holding  in  solution  ferrite  in 
hypo-eutectoid  steel  and  cementite  in  hyper-eutectoid  steel. 


4  LESSON   XIII  — THE   HARDENING   OF   STEEL 

Occurrence  of  Austenite.  —  While  present  in  all  steels  above  their  critical  range 
austenite  is  very  rarely  found  in  ordinary  steels  cooled  to  atmospheric  tempera- 
ture. This  is  due  to  the  rapidity  with  which  austenite  is  transformed  on  cooling 
through  the  critical  range  if  not  into  an  aggregate  of  ferrite  and  cementite,  at  least 
into  some  transition  stages.  In  the  commercial  hardening  of  ordinary  carbon  steel 
the  passage  of  the  metal  through  its  range  is  never  sufficiently  rapid  to  retain  in  the 
cold  a  small  amount  even  of  undecomposed  austenite.  To  prevent  the  transforma- 
tion of  a  portion  of  the  austenite  the  conditions  generally  affecting  the  hardening  of 
the  metal  must  be,  so  to  speak,  greatly  intensified:  (1)  the  steel  should  be  highly  car- 
burized,  (2)  quenching  should  be  from  a  high  temperature  (1000  deg.  C.  or  more), 
and  (3)  a  very  effective  quenching  bath  should  be  used  such  as  ice-cold  water.  In 


Fig.  3. — Steel.     Carbon  1 .57  per  cent.    Magnification  not  stated.    Heated  to 
1050  deg.  C.  and  quenched  in  ice-water.     (Osmond.) 


Figure  2  is  shown,  after  Osmond,  the  structure  of  steel  containing  1.57  per  cent  carbon 
heated  to  1050  deg.  C.  and  quenched  in  ice-water.  The  magnification  is  1000  diam- 
eters. The  dark-colored,  zigzag  constituent  is  martensite;  the  light  matrix,  or  back- 
ground, is  austenite.  The  structure  of  the  same  steel,  under  lower  magnification,  is 
seen  in  Figure  3.  By  the  drastic  quenching  treatment  just  described  it  is  possible, 
in  the  case  of  high  carbon  steel,  to  retain  more  than  one  half  of  the  steel  in  its  aus- 
tenitic  condition. 

The  retention  of  austenite  in  the  cold  is  greatly  helped  by  the  presence  of  some 
elements  such  as  manganese  and  nickel  which  lower  the  position  of  the  transforma- 
tion range,  eventually  depressing  it  below  atmospheric  temperature  and,  therefore, 
causing  the  steel  to  remain  austenitic  even  after  slow  cooling.  This  actually  takes 
place  in  the  presence  of  some  12  per  cent  manganese  or  25  per  cent  nickel.  When 
high  carbon  steel  contains  over  one  per  cent  of  manganese  it  can  be  retained  in  its  aus- 


LESSON  XIII  — THE  HARDENING  OF  STEEL  5 

tonitic  condition  upon  very  rapid  cooling  through  its  range.  In  Figure  4,  for  instance, 
is  seen,  after  Robin,  the  structure  of  a  very  small  sample  of  steel  (1  to  2  cubic  centi- 
meters) containing  from  1.5  to  1.7  per  cent  carbon  and  one  per  cent  manganese 
quenched  in  ice-cold  water  from  a  temperature  of  1400  deg.  C.  It  consists  entirely  of 


Fig.  4.  —  Steel.  Carbon  1.60  percent,  manganese  1.00  per 
cent.  Magnified  300  diameters.  Heated  to  1400  deg  C.  and 
quenched  in  ice-cold  water  (Robin.) 


Fig.  5.  —  Steel.  Carbon  1.94  per  cent,  manganese  2.20  per  cent. 
Heated  to  1100  deg.  C.  and  quenched  in  ice-cold  water. 
(Maurer.) 


austenite.  Maurer,  likewise,  succeeded  in  retaining  in  its  austenitic  condition  a  steel 
containing  2  per  cent  manganese  and  2  per  cent  carbon  by  quenching  it  in  ice-cold 
water  from  a  temperature  of  1100  deg.  (Fig.  5).  As  the  manganese  increases  the 
retention  of  austenite  becomes  easier,  that  is,  the  quenching  need  not  be  so  drastic  nor 
the  carbon  content  so  high.  Finally  with  10  or  more  per  cent  of  manganese  and  one 


6  LESSON  XIII  — THE  HARDENING  OF  STEEL 

or  more  per  cent  carbon  the  steel  remains  austenitic  after  slow  cooling.  The  struc- 
ture and  properties  of  manganese  steel  will  be  considered  in  another  lesson. 

To  sum  up:  (1)  austenite  is  never  produced  in  the  commercial  hardening  of 
ordinary  carbon  steel;  (2)  it  may  be  retained  in  the  cold,  however,  associated 
with  considerable  martensite  in  quenching  very  high  carbon  steel,  from  a  very  high 
temperature  in  ice-cold  water,  as,  for  instance,  by  quenching  steel  containing  not  less 
than  1.50  per  cent  carbon  from  1000  deg.  C.  or  higher;  (3)  in  the  presence  of  1  per 
cent  of  manganese  very  small  pieces  of  very  high  carbon  steel  may  be  retained  wholly 
in  their  austenitic  condition  by  quenching  them  from  a  very  high  temperature,  as, 
for  instance,  by  quenching  in  ice-cold  water  from  1400  deg.  C.  small  pieces  of  steel 
containing  1  per  cent  of  manganese  and  .not  less  than  1.5  per  cent  carbon  (Robin); 
(4)  with  increasing  proportions  of  manganese  the  transformation  of  austenite  may 
be  prevented  in  steel  containing  less  carbon  and  quenched  from  lower  temperatures 
(Maurer);  (5)  manganese  steels  containing,  for  instance,  10  or  more  per  cent  manganese 
and  one  or  more  per  cent  carbon  remain  austenitic  after  slow  cooling;  (6)  nickel 
steel  containing  some  25  per  cent  of  nickel  likewise  remains  austenitic  on  slow  cooling. 

Benedicks  contends  that  in  the  preservation  of  austenite  in  carbon  steel  by 
rapid  cooling  an  important  part  is  played  by  the  very  great  pressure  to  which  the 
metal  is  subjected,  (1)  because  of  the  shrinkage  of  the  exterior  portion  or  outer  shell 
on  the  interior  and  (2)  because  of  the  dilatation  accompanying  the  change  from 
gamma  to  beta  iron.  Were  it  not  for  this  pressure  Benedicks  believes  that  the  trans- 
formation of  austenite  could  not  be  prevented.  As  an  evidence  of  this  he  shows  that 
austenitic  steels  produced  by  quenching  are  austenitic  only  in  their  interior,  i.e.  where 
the  pressure  had  been  greatest,  the  outside  layers  in  which  the  pressure  was  small  or 
nil  being  martensitic.  He  shows,  further,  that  on  removing  by  grinding  the  marten- 
sitic  shell  the  austenitic  core,  in  turn,  becomes  martensitic  owing  to  the  removal  of 
the  pressure  exerted  upon  it  by  that  shell.  Again  the  quenching  of  steel  cylinders 
surrounded  by  cast  iron  shells  resulted  in  the  formation  of  austenite  close  to  the 
skin  of  the  steel  cylinders  owing  apparently  to  the  very  great  pressure  exerted  upon 
the  steel  by  the  contraction  of  the  iron  shells. 

Etching  of  Austenite.  —  The  etching  reagents  usually  applied  to  bring  out  the 
structure  of  unhardened  steel,  namely,  picric  acid,  nitric  acid,  tincture  of  iodine,  etc., 
do  not  always  yield  satisfactory  results  in  the  case  of  hardened  steel.  Kourbatoff 
discovered  a  complex  reagent  which  often  produces  greater  contrasts  between  the 
various  constituents.  It  is  made  up  by  mixing  one  part  of  amyl  alcohol,  one  part  of 
ethyl  alcohol,  one  part  of  methyl  alcohol,  and  one  part  of  a  4  per  cent  solution  of 
nitric  acid  in  acetic  anhydride  and  should  be  prepared  just  before  use. 

Heyn  recommends  for  etching  hardened  steel  a  solution  containing  one  part  of 
hydrochloric  acid  and  99  parts  of  absolute  alcohol.  More  uniform  results  are  ob- 
tained if  a  weak  current  of  electricity  be  passed  through  the  solution,  the  samples  to 
be  etched  forming  the  positive  pole  while  the  negative  electrode  may  consist  con- 
veniently of  a  piece  of  sheet  lead.  With  the  assistance  of  the  electric  current  the  use 
of  a  very  dilute  aqueous  solution  is  advisable,  namely,  one  part  of  hydrochloric  acid 
in  500  parts  of  distilled  water. 

Osmond,  likewise,  used  successfully  a  solution  of  10  per  cent  of  hydrochloric  acid 
in  water  by  which  the  martensite  is  colored  darker  than  austenite,  the  treatment  re- 
quiring several  minutes.  Osmond  writes:  "There  is  more  regularity  obtained  by 
having  the  specimen  connected,  by  means  of  a  platinum  wire,  with  the  positive  pole 
of  a  bi-chromate  cell,  a  strip  of  platinum  placed  in  the  acid  being  connected  with  the 


LESSON   XIII  — THE   HARDENING   OF  STEEL  7 

negative  pole.  In  this  way  the  specimen  becomes  the  anode,  and  the  platinum  the 
cathode." 

Benedicks  recommends  for  the  etching  of  martenso-austenitic  steel  a  5  per  cent 
alcoholic  solution  of  metanitrobenzol-sulphonic  acid  which  always  darkens  martensite 
more  than  austenite.  Immersions  of  some  fifteen  seconds  are  generally  sufficient. 

Structure  of  Austenite.  —  When  austenite  and  martensite  occur  in  the  same  sam- 
ple the  latter  is  generally  colored  darker  than  the  former  (Figs.  2  and  3).  Martensite, 
moreover,  is  readily  distinguishable  because  of  its  zigzag  or  needle  shape.  Some 
writers  claim  that  martensite  is  sometimes  colored  less  than  austenite.  Indeed  Maurer 
contends  that  this  is  always  so,  arguing  that  if  most  photomicrographs  indicate  the 
contrary  it  is  because  the  martensite  had. undergone  a  certain  amount  of  tempering 
resulting  in  the  formation  of  some  troostite  as  later  explained.  According  to  this 
writer,  in  order  to  prevent  any  tempering  of  the  martensite,  and  therefore  the  forma- 
tion of  dark-colored  troostite,  great  care  must  be  exercised  in  sawing,  polishing,  etc. 
To  this  Benedicks  replies  that  it  cannot  always  be  so  for  in  quenching  austenite  in 
liquid  air  martensite  is  formed  which  must  be  free  from  troostite  and  which,  never- 
theless, is  darker  than  austenite.  It  may  be  asked,  however,  whether  it  is  certain 
that  the  martensite  produced  in  this  way  is  actually  free  from  troostite.  Evidences 
of  a  more  conclusive  nature  are  needed  to  account  satisfactorily  for  the  shifting  in 
the  relative  coloration  of  austenite  and  martensite  when  occurring  side  by  side.  Pure 
austenite  is  made  up  of  polyhedric  grains l  (see  Figs.  4  and  5)  which,  as  explained  in 
previous  lessons  in  connection  with  the  structure  of  gamma  iron,  are  undoubtedly 
made  up  of  true  crystals,  small  octahedra  according  to  Osmond.  It  should  be  noted 
that  when  austenite  occurs  in  the  presence  of  much  martensite  (Fig.  2)  its  polyhedric 
structure  is  not  brought  out.  Twinnings  are  frequently  observed  in  austenite  (see 
Lesson  II,  Fig.  11)  although  it  has  been  claimed  that  they  form  only  after  straining, 
especially  if  followed  by  annealing. 

Baykoff  succeeded  in  etching  austenite  above  the  critical  range  of  the  steel,  that 
is,  in  a  range  of  temperature  where  it  is  stable.  He  accomplished  this  by  heating 
polished  steel  samples  in  a  porcelain  tube  through  which  a  current  of  hydrogen  was 
kept  circulating  and  by  passing  through  it  gaseous  hydrochloric  acid  when  the  de- 
sired temperature  had  been  obtained.  The  resulting  structures  were  found  to  be 
polyhedric  even  in  the  presence  of  very  little  carbon,  thus  confirming  the  previous 
belief  as  to  the  crystalline  character  of  austenitp. 

Properties  of  Austenite.  —  Since  the  carbon  content  of  austenite  varies  from  a 
mere  trace  to  nearly  2  per  cent  it  may  well  be  expected  that  its  physical  properties 
will  likewise  vary,  i.e.  that  it  will  increase  in  hardness  and  strength  and  decrease  in 
ductility  as  the  carbon  increases.  Osmond  has  shown  conclusively  that  austenite 
was  softer  than  martensite  of  identical  carbon  content.  When  it  is  remembered  that 
in  order  to  produce  austenite  in  ordinary  carbon  steel  all  the  factors  generally  in- 
creasing the  hardness  of  the  metal  must  be  intensified,  it  is  at  first  surprising  that 
the  energetic  quenching  treatment  required  should  yield  a  softer  metal.  The  con- 
clusion must  be  that  gamma  iron  is  softer  than  beta  iron.  This  relative  softness  of 
austenite  is  well  shown  by  Osmond  in  Figure  6  which  represents  the  structure  of  a 
bar  of  steel  containing  1.55  per  cent  carbon  in  the  center  and  a  gradually  decreasing 
amount  towards  the  outside.  This  bar  was  heated  to  1050  deg.  C.  and  quenched  in 

1  Because  of  this  structure  Guillet  and  some  other  writers  refer  to  steels  composed  of  austenite 
:i-  ••polyhedric"  steels.  This  does  not  seem  advisable  as  it  may  lead  to  confusion,  for  other  steels 
also  have  polyhedric  structures,  to  wit,  very  low  carbon  (ferritic)  steels. 


8 


LESSON   XIII  — THE   HARDENING   OF   STEEL 


mercury  at  a  temperature  of  —9  cleg.  C.  After  polishing  hut  before  etching  a  needle 
was  repeatedly  drawn  across  it  from  end  to  end  with  even  pressure.  The  photograph 
clearly  shows  that  the  needle  scratched  the  steel  (1)  where  it  contains  so  little  carbon 
(0.40  to  0.60  per  cent)  that  it  was  only  partly  martensitic  and  hence  relatively  soft, 
(2)  in  those  regions  which  because  of  very  high  carbon  content  (1.30  to  1.55  per  cent) 

Carbon 
per  cent 

0.8C 
0.90 
1.00 
1.10 
1.20 
1.30 
1.40 


1.50 
1.55 
1.50 
1.40 
1.30 
1.20 
1.10 
1.00 
0.90 
0.80 
0.70 
0.60 
0.50 

0.40 
0.36 


0.35 


.  Hardenite  and 
Martens!  te 


Austenite  and 
Hardenite 


Hardenite  and 
'  Martensite 


Fig.  6. — Showing  the  relative  softness 
of  austenite. 

were  partly  austenitic,  and  (3)  that  it  failed  to  scratch  it  in  those  regions  which  because 
of  a  more  moderate  amount  of  carbon  (0.70  to  1.20  per  cent)  were  fully  martensitic 
and,  therefore,  very  hard.  It  is  also  well  known  that  high  carbon  austenitic  manga- 
nese steel,  while  extremely  difficult  to  machine,  can  be  readily  scratched  by  a  needle, 
being  mineralogically  softer,  therefore,  than  high  carbon,  martensitic  steel.  Rosen- 
hain  and  Humphrey  have  shown  that  above  the  critical  range  austenite  (gamma 
iron)  was  much  softer  than  beta  iron.  Since  steel  above  its  critical  range  is  non- 


LESSON   XIII  — THE   HARDENING   OF  STEEL  9 

magnetic  we  should  expect  steels  which  remain  austenitic  in  the  cold  to  be  non-mag- 
netic. This  we  know  to  be  the  case,  for  manganese  as  well  as  nickel  austenitic  steels 
are  non-magnetic. 

Some  of  the  physical  properties  of  austenite  may  be  inferred  from  the  known 
properties  of  austenitic  steels  such  as  manganese  and  high  nickel  steels.  These  are 
known  to  be  very  ductile  (after  suitable  heat  treatment),  tenacious,  of  low  elastic 
limit,  to  possess  very  high  resistance  to  wear  although  their  mineralogical  hardness  is 
not  excessive  and  to  be  machined  only  with  great  difficulty.  They  have,  like  gamma 
iron,  a  very  high  electrical  resistance. 

It  has  already  been  pointed  out  that  the  crystallization  of  austenite  is  probably 


Fig.  7.  —  Austenitic  steel  quenched  in  liquid  air.     Magnified  250 
diameters.     (Osmond.) 


cubic,  the  octahedron  being  its  prevailing  crystalline  form.  Le  Chatelicr,  however, 
believes  that  austenite  crystallizes  in  the  orthorhombic  system  with  octahedral  cleav- 
age. On  slow  cooling  through  the  critical  range  in  the  absence  of  considerable  quan- 
tities of  retarding  elements  such  as  manganese  and  nickel,  austenite  rejects  a  sufficient 
amount  of  ferrite  in  hypo-eutectoid,  or  of  cementite  in  hyper-eutectoid,  steel  to  assume 
the  eutectoid  composition  (0.85  per  cent  C.  or  thereabout)  when  it  is  converted  bodily 
into  pearlite.  This  transformation  is  not  sudden,  however,  several  transition  con- 
stituents being  formed,  namely,  martensite,  troostite,  and  sorbite. 

It  will  be  seen  in  another  lesson  that  on  tempering  austenite,  that  is,  on  reheating 
it  below  the  critical  range  of  the  metal  it  is  likewise  converted  gradually  and  succes- 
sively into  martensite,  troostite,  and  sorbite  or  according  to  some  writers  directly 
into  troostite  and  then  into  sorbite. 

Quenching  austenite  in  liquid  air  results  in  the  formation  of  martensite  with  in- 
creased volume  causing  swellings  of  the  polished  surface  as  shown  in  Figure  7. 


10  LESSON   XIII  — THE   HARDENING   OF  STEEL 

MARTENSITE 

Nature  of  Martensite.1  —  It  is  very  generally  believed  that  martensite  corres- 
ponds to  an  early  stage  in  the  transformation  of  austenite  in  passing  through  the 
critical  range.  Opinions  differ,  however,  as  to  its  exact  nature.  Accepting  the  possi- 
bility of  iron  existing  under  three  allotropic  forms,  namely,  as  gamma,  beta,  and  alpha 
iron,  and  the  carbon  under  two  distinct  conditions,  namely,  as  the  crystallized  car- 
bide FesC  or  cement  carbon  and  of  this  carbide  or  possibly  elementary  carbon  being 
dissolved  in  iron,  i.e.  as  hardening  carbon,  what  are  the  probable  conditions  of  these 
two  constituents  in  martensite?  Osmond  and  many  others  believe  that  in  martensite 
iron  is  present  chiefly  in  its  beta  condition,  holding  carbon  in  solution,  hence  the 
great  hardness  of  that  constituent.  Since  martensite  is  magnetic,  however,  it  must 
also  contain  an  appreciable  quantity  of  magnetic  alpha  iron.  This  theory,  which  may 
be  called  the  allotropic  theory,  is  the  one  most  widely  held.  Le  Chatelier,  not  believ- 
ing in  the  existence  of  beta  iron,  considers  martensite  as  essentially  a  solid  solution  of 
carbon  in  alpha  iron,  owing  its  great  hardness  to  its  state  of  solid  solution  and  its 
magnetism  to  the  presence  of  alpha  iron.  Edwards  contends  that  austenite  and 
martensite  are  in  reality  the  same  constituent,  namely,  a  solid  solution  of  carbon  in 
gamma  iron,  differing  only  in  structural  aspect,  the  needles  of  martensite  resulting 
from  the  twinning  of  austenite  caused  by  the  severe  pressure  exerted  upon  it  during 
rapid  cooling.  He  bases  his  view  chiefly  upon  the  absence  of  the  point  A2  in  medium 
high  and  high  carbon  steels,  from  which  he  infers  that  beta  iron  does  not  form  in 
those  steels,  losing  sight  of  the  fact  that  the  points  As.2  and  Aa.2.i  may  very  well, 
and  probably  do,  include  the  A2  changes.  Kroll  also  speaks  of  martensite  as  repre- 
senting the  "mutilated  structure  of  austenite  due  to  twinning."  Arnold  believes  that 
martensite  is,  like  austenite,  the  carbide  Fe24C  holding  in  solution  ferrite  in  hypo- 
eutectoid  steel  and  cementite  in  hyper-eutectoid  steel. 

Careful  consideration  of  the  evidences  at  hand  leads  to  the  adoption  of  the  first 
theory  (Osmond's)  as  the  one  best  supported.  That  martensite  is  to  a  great  extent 
a  solid  solution  seems  evident  from  the  fact  that  it  contains  a  great  deal  of  hardening, 
i.e.  dissolved  carbon,  and  it  seems  probable  that  beta  iron  is  the  solvent,  for  if  gamma 
iron  were  the  solvent  it  would  not  explain  the  greater  hardness  of  martensite  com- 
pared to  that  of  austenite  while  we  have  good  reason  to  doubt  the  power  of  alpha  iron 
to  dissolve  carbon  seeing  that  below  the  critical  range,  i.e.  when  in  its  alpha  form, 
iron  will  not  absorb  carbon. 

Occurrence  of  Martensite.  —  Martensite  is  most  readily  obtained  through  the 
quenching  of  small  pieces  of  high  carbon  steel  in  cold  water;  in  the  case  of  largo 
pieces,  while  the  outside  portion  may  be  martensitic  their  center  is  likely  to  be  partly 
troostitic.  In  low  carbon  steel  it  is  more  difficult  still  to  prevent  the  formation  of 
some  troostite  while  in  steel  containing  very  little  carbon  free  ferrite  as  well  is  likely 
to  be  present.  In  very  high  carbon  steel  some  free  cementite  is  generally  associated 
with  the  martensite. 

Etching  of  Martensite.  —  Dilute  alcoholic  solutions  of  picric,  nitric,  or  hydro- 
chloric acid  generally  bring  out  satisfactorily  the  structure  of  martensite  but  the 
Kourbatoff  reagent,  already  described,  sometimes  yields  better  results.  Martensite 
generally  darkens  more  quickly  than  austenite  but  always  remains  much  lighter  than 
troostite. 

1  This  name  was  selected  by  Osmond  in  honor  of  A.  Marteas  a  distinguished  German  metallur- 
gist and  testing  engineer. 


LESSON   XIII  — THE   HARDENING   OF  STEEL  11 

Structure  of  Martensite.  —  Martensitic  structures  are  shown  in  Figures  1  and  8. 
Osmond  describes  the  structure  of  martensite  as  consisting  of  three  systems  of  fibers, 
respectively  parallel  to  the  three  sides  of  a  triangle  and  crossing  each  other  frequently. 
Osmond  also  states  that  when  the  metal  contains  less  carbon  the  needles  are  longer 
and  more  clearly  differentiated,  other  things  being  equal.  According  to  crystallog- 
raphers  these  markings,  in  reality  cleavages  of  octahedra,  indicate  crystallites  of 
the  cubic  system  and,  therefore,  afford  an  additional  evidence  of  the  cubic  crystalliza- 
tion of  austenite  from  which  martensite  is  derived.  Osmond  and  Cartaud  refer  to 
them  as  probable  pseudomorphs  of  twinnings  due  to  tension,  occurring  in  gamma 
iron  through  partial  formation  of  the  bulky  beta  and  alpha  modifications. 

Properties  of  Martensite.  —  The  carbon  content  of  martensite  varies  from  a  mere 
trace  to  as  much  as  one  per  cent,  and  possibly  more,  in  very  suddenly  cooled  hyper- 
eutectoid  steels.  In  high  carbon  steels,  however,  it  is  difficult  to  prevent  the  setting 


Fig.  8.  —  Steel.  Carbon  1.25  per  cent.  Mag- 
nified 150  diameters.  Heated  to  1232  deg. 
C.  and  quenched  in  oil.  (C.  C.  Buck,  Cor- 
respondence Course  student.) 

free  of  much  of  the  excess  cementite  even  on  very  quick  cooling.  From  this  varia- 
tion of  its  percentage  of  carbon  it  follows  that  the  properties  of  martensite  must  also 
vary.  As  the  carbon  increases  its  hardness  and  strength  increase  while  its  ductility 
decreases,  martensitic  steels  being  generally  hard  and  brittle  and,  therefore,  unforge- 
able  in  the  cold. 

It  will  be  seen  in  another  lesson  that  on  heating  martensite  below  the  critical 
range,  i.e.  on  tempering  it,  it  is  converted  first  into  troostite  and  then  into  sorbite. 

TROOSTITE 

Nature  of  Troostite.1  —  While  most  writers  believe  that  troostite  represents  a  con- 
dition of  the  steel  resulting  from  the  transformation  of  martensite  and,  therefore,  a 
further  step  in  the  transformation  of  austenite,  much  difference  of  opinion  exists  as 
to  its  exact  nature.  The  controversy  has  given  rise  to  a  very  large  and  apparently 
exaggerated  amount  of  discussion.  Here,  as  in  the  case  of  martensite,  we  must  con- 

1  The  name  troostite  was  selected  by  Osmond  in  honor  of  the  French  chemist  Troost. 


12  LESSON  XIII  — THE  HARDENING  OF  STEEL 

sider  the  possibility  of  the  iron  existing  in  the  gamma,  beta,  or  alpha  form  or  in  two 
or  even  all  three  of  these  conditions,  while  the  carbon  may  exist  as  cement  carbon  or 
as  hardening  carbon  or  partly  as  cement  and  partly  as  hardening  carbon.  Then 
the  association  between  iron  and  carbon  may  be  of  the  nature  of  an  aggregate  or  of  a 
solid  solution  or  partly  aggregate  and  partly  solution,  or,  indeed,  half  way  between 
aggregate  and  solution,  namely,  resembling  a  colloidal  solution,  an  emulsion,  or  an 
^uncoagulated  substance.  Nearly  every  conceivable  hypothesis  has  been  suggested  to 
account  for  the  nature  of  troostite.  It  has  been  described  as  a  solid  solution  of 
carbon  or  of  carbide  in  gamma  iron,  in  beta  iron,  and  in  alpha  iron.  It  has  also  been 
suggested  that  it  might  be  pure  beta  iron. 

In  later  years,  thanks  chiefly  to  the  enlightening  discussions  of  Benedicks  sup- 
ported by  the  weighty  evidence  of  skilfully  conducted  experiments,  metallographists 
have  come  to  regard  troostite  as  an  uncoagulated  mixture  of  the  constituents  of  mar- 
tensite  and  sorbite,  that  is,  of  (1)  carbide  dissolved  in  beta  iron,  (2)  crystallized  Fe3C. 
and  (3)  crystallized  alpha  iron  —  clearly  martensite  passing  to  sorbite.  Benedicks 
compares  it  to  a  colloidal  solution1  while  Arnold  had  previously  described  it  as 
"emulsified"  pearlite.2  The  existence  of  considerable  dissolved  (hardening)  carbon 
in  troostite  is  proven  by  analysis  as  well  as  the  existence  of  considerable  crystallized 
Fe3C  (cement  carbon).  Its  relatively  great  hardness  points  strongly  to  the  presence 
of  a  considerable  amount  of  beta  iron  while  its  magnetism  demands  the  presence  of 
alpha  iron.  Benedicks'  hypothesis  is  consistent  with  what  we  know  of  the  formation 
of  troostite  and  of  its  properties. 

In  the  report  of  the  Committee  on  the  Nomenclature  of  the  Microscopical  Con- 
stituents of  Iron  and  Steel  of  the  International  Association  for  Testing  Materials, 
troostite  is  defined  as  follows:  "probably  aggregate.  In  the  transformation  of  aus- 
tenite,  the  stage  following  martensite  and  preceding  sorbite  .  .  .  An  uncoagulated 
conglomerate  of  the  transition  stages." 

Occurrence  of  Troostite.  —  In  order  to  produce  troostite  on  cooling  steel  from 
above  its  critical  range,  it  is  necessary  that  the  cooling  through  the  range  should 
be  so  regulated  as  to  allow  it  to  form  and  at  the  same  time  prevent  its  further 
transformation  (into  sorbite  and  pearlite).  These  conditions  may  prevail  (1)  in  cool- 
ing slowly  to  the  middle  of  the  range,  thus  permitting  the  formation  of  troostite  (see 
Fig.  13),  and  then  quickly  to  atmospheric  temperature,  thus  retaining  troostite,  and 
(2)  in  cooling  through  the  range  at  a  rate  uniform  throughout  but  so  regulated  as  to 
cause  the  production  and  retention  of  troostite  (Fig.  13)  as,  for  instance,  quenching 
large  pieces  in  water  when  the  central  portions  at  least  will  be  troostitic,  or  quench- 
ing smaller  pieces  in  oil.  It  will  be  explained  in  the  next  lesson  that  troostite  may 
also  be  produced  by  tempering  (i.e.  reheating  below  the  critical  range)  austenitic 
and  martensitic  steels. 

Troostite  is  readily  produced  by  heating  a  bar  of  steel,  containing  0.50  per  cent 
carbon  or  more,  white  hot  at  one  end  and  quenching  it  in  water,  when  at  some 
distance  from  the  heated  end  the  temperature  must  necessarily  have  been  such  as  to 
produce  troostite.  This  can  generally  be  detected  by  means  of  a  file,  the  martensitic 

1  A  colloid  may  be  regarded  as  a  substance  passing  from  the  state,  of  solution  to  that  of  an 
aggregate  or  vice  versa;  it  is  no  longer  a  solution  but  not  yet  an  aggregate.    To  express  it  more  scien- 
tifically, while  not  a  true  solution  the  particles  of  solvent  and  solute  are  ultra-microscopic.    Accord- 
ing to  Le  Chatelier  so-called  colloidal  solutions  are  in  no  way  solutions,  but  merely  liquids  holding 
in  suspension  very  finely  divided  particles;  the  expression,  he  says,  should  not' be  used. 

2  "Emulsified  carbide  present  in  an  excessively  fine  state  of  division  in  tempered  steels."    (1895.) 


LESSON   XIII  — THE   HARDENING   OF   STEEL 


13 


portion  of  the  bar  being  too  hard  to  be  marked  while  the  troostitic  part,  although 
hard,  can  be  scratched.  The  sorbitic  and  pearlitic  portions  are  decidedly  softer. 

Properties  of  Troostite.  —  It  will  be  obvious  from  the  foregoing  description  of  the 
nature  and  formation  of  troostite  that  its  physical  properties  must  be  intermediate 
between  those  of  martensite  and  of  sorbite.  For  like  carbon  content  troostite  is 
softer  and  more  ductile  than  martensite  but  harder  and  less  ductile  than  sorbite.  It 
will  be  shown  that  at  some  400  deg.  C.  it  begins  to  be  transformed  into  sorbite. 

Etching  of  Troostite.  —  Troostite  is  colored  decidedly  darker  than  any  other  con- 
stituent by  the  ordinary  etching  reagents.  While  dilute  alcoholic  solutions  of  nitric, 
picric,  or  hydrochloric  acid  yield  satisfactory  results,  Kourbatoff  s  reagent  is  pre- 
ferred by  some. 

Structure  of  Troostite.  —  Troostite  generally  occurs  as  dark-colored,  irregular 
areas,  representing  sections  through  nodules  generally  accompanied  by  martensite  or 


Fig.  9.  —  Steel.  Quenched  during  critical  range. 
Magnified  200  diameters.  Slightly  etched. 
Troostite  and  martensite.  (Guillet.) 


Fig.  10.  —  Steel.  Carbon  0.4o  per  cent.  Magni- 
fied 1000  diameters.  Troostite  and  martensite. 
(Osmond.) 


sorbite  or  both  or  as  membranes  surrounding  martensite  grains  (Figs.  9  to  12).  In 
hypo-eutectoid  steel  free  ferrite,  and  in  hyper-eutectoid  steel  free  cementite,  may  also 
be  present  and,  indeed,  even  well-developed  pearlite  (Fig.  12).  Osmond  describes  the 
structure  of  troostite  as  "almost  amorphous,  slightly  granular,  and  mammilated." 

Sorbite.  —  Sorbite  is  not,  properly  speaking,  a  constituent  of  hardened  steel.  It 
seems  appropriate,  however,  to  again  mention  it  here  seeing  that,  it  constitutes  the 
connecting  link  between  annealed  (pearlitic)  steels  and  hardened  (troosto-marten- 
sitic)  steels,  and  also  because  it  results  from  the  transformation  of  troostite,  thus  com- 
pleting the  various  stages  assumed  by  iron  carbon  alloys  in  passing  from  the  condition 
of  austenite,  stable  above  the  range,  to  that  of  pearlite,  stable  below  that  range. 
These  stages  are  (1)  austenite,  (2)  martensite,  (3)  troostite,  (4)  sorbite,  and  (5) 
pearlite. 

Sorbite  is  now  generally  regarded  as  an  uncoagulated  mixture  of  the  constituents 
of  troostite  and  of  pearlite;  it  apparently  contains  (1)  some  hardening  carbon,  i.e. 
carbon  or  FesC  dissolved  in  beta  iron,  hence  the  greater  hardness  and  strength  of 
sorbite  compared  to  the  hardness  and  strength  of  pearlite,  (2)  a  considerable  quan- 


14 


LESSON  XIII  — THE  HARDENING  OF  STEEL 


tity  of  alpha  iron,  hence  its  magnetism  and  relative  softness,  and  (3)  a  considerable 
quantity  of  crystallized  FesC  (cement  carbon)  as  proven  by  analysis.  While  sorbite 
probably  contains  the  same  constituents  as  troostite  it  holds  considerably  less  unde- 
composed  solid  solution  and  considerably  more  alpha  iron,  hence  it  is  much  softer 


Fig.  11.  —  Steel.  Carbon  0.54  per  cent.  Magni- 
fied 100  diameters.  Troostite  and  martensite. 
(Boynton.) 


Fig.  12.  —  Steel.    Carbon  0.54  per  cent.    Magnified  1000  diameters.    Martensite,  troostite, 

sorbite,  and  pearlite.    (Boynton.) 

and  more  ductile  than  troostite.  In  other  words  the  transformation  which  eventually 
must  lead  to  the  formation  of  pearlite  is  more  advanced  in  sorbite  than  it  is  in  troost- 
ite. The  nomenclature  committee,  already  referred  to,  describes  sorbite  as  follows: 
"Aggregate  ...  In  the  transformation  of  austenite,  the  stage  following  troostite 
.  .  .  and  preceding  pearlite.  Most  writers  believe  it  essentially  an  uncoagulated 
conglomerate  of  irresoluble  pearlite  with  ferrite  in  hypo-  and  cementite  in  hyper- 
eutectoid  steels  respectively." 


LESSON  XIII  — THE  HARDENING  OF  STEEL  15 

The  occurrence,  etching,  structure,  and  properties  of  sorbite  have  been  described 
in  Lessons  XI  and  XII  when  it  was  shown  that  it  is  formed  (1)  in  small  pieces  of  steel 
cooling  in  the  air  from  above  their  critical  range,  (2)  in  larger  pieces  quenched  in  oil 
from  above  the  range,  or  (3)  in  small  pieces  quenched  in  water  from  near  the  bottom 
of  the  range.  In  other  words  to  form  sorbite  we  must  so  regulate  the  cooling  through 
the  critical  range  that  it  is  allowed  to  form  but  prevented  from  further  transforma- 
tion (into  pearlite).  It  will  be  seen  in  the  next  lesson  that  sorbite  is  also  formed  on 
tempering  austenitic,  martensitic,  and  troostitic  steels. 

By  its  physical  properties  sorbite  occupies  an  intermediate  position  between 
troostite  and  pearlite;  as  previously  mentioned  it  is  stronger,  harder,  and  less  ductile 
than  pearlite  but  softer  and  more  ductile  than  troostite. 

Troosto-Sorbite.  —  Kourbatoff  gives  the  name  of  troosto-sorbite  to  a  constituent 
associated  with  martensite  and  austenite  in  quenching,  from  a  high  temperature,  steels 
very  high  in  carbon.  It  is  not  clear  that  this  constituent  is  more  than  a  mixture  of 
troostite  and  sorbite.  We  may  talk  of  troosto-sorbite  as  we  do  of  a  greenish  blue 
tint  to  indicate  shades  intermediate  between  green  and  blue,  and  similarly  the  ex- 
pressions martenso-austenite,  troosto-martensite,  and  sorbitic-pearlite,  or  like  expres- 
sions, are  useful  and  their  meanings  obvious.  In  the  report  of  the  Committee  on  the 
Nomenclature  of  the  Microscopical  Constituents  of  Iron  and  Steel  of  the  International 
Association  for  Testing  Materials,  troosto-sorbite  is  thus  denned:  "Indefinite  aggre- 
gate, the  troostite  and  the  sorbite  which  lie  near  the  boundary  which  separates  these 
two  aggregates." 

Hardenite.  —  The  name  of  hardenite  is  frequently  given  both  to  austenite  and  to 
martensite  of  eutectoid  composition,1  i.e.  to  the  original  austenite  of  eutectoid  steel 
and  to  the  residual  austenite  of  hypo-  and  hyper-eutectoid  steel  after  rejection  of  the 
full  amount  of  free  ferrite  or  of  free  cementite.  In  other  words  the  name  is  applied 

(1)  to  the  condition  of  austenite  in  slowly  cooled  steels  immediately  preceding  its 
conversion  into  martensite  and  (2)  to  the  resulting  martensite  (necessarily  of  eutec- 
toid composition  if  the  cooling  to  the  range  has  been  sufficiently  slow).     It  is  unfor- 
tunate that  the  same  term  is  used  to  designate  both  austenite  and  martensite,  two 
apparently  sharply  different  constituents,  as  it  is  likely  to  lead  to  confusion.    Its  use 
should  be  restricted  to  the  designation  of  austenite  of  eutectoid  composition.    Giving 
it  this  meaning  it  will  be  apparent,  as  later  explained,  that  hardenite  possesses  maxi- 
mum hardening  power  and,  therefore,  that  steel  made  up  exclusively  of  hardenite,  i.e. 
eutectoid  steel,  possesses  maximum  hardening  power. 

Rate  of  Cooling  through  Critical  Range  vs.  Structure  of  Steel.  —  It  has  been  made 
clear  in  the  foregoing  pages  (1)  that  in  order  to  retain  some  austenite  in  the  cold  the 
metal  should  be  highly  earburized  and  very  quickly  cooled  from  a  high  temperature, 

(2)  that  pearlite  is  produced  by  very  slow  cooling  through  the  critical  range,  and 

(3)  that  in  order  to  cause  the  formation  of  any  of  the  three  recognized  transition  con- 
stituents, namely,  martensite,  troostite,  and  sorbite,  the  steel  should  be  cooled  through 
its  critical  range  in  such  a  way  as  to  allow  the  formation  of  the  desired  constituent 
while  preventing  its  further  transformation  as,  for  instance,  (a)  by  cooling  the  metal 
slowly  to  that  portion  of  the  range  in  which  the  constituent  is  formed  and  then  quickly 
to  atmospheric  temperature  or  (6)  by  cooling  the  metal  through  its  range  at  a  uni- 

1  Originally  the  name  hardenite  was  applied  by  Howe  to  austenite  and  martensite  of  any  com- 
position (18S8).  Osmond  used  it  to  designate  austenite  saturated  with  carbon  (1897).  Both  these 
m>;aninu;s  have  been  withdrawn  by  their  proposers.  Arnold  calls  hardenite  the  carbide  Fe^C  which 
he  believes  exists  above  the  critical  range. 


16 


LESSON   XIII  — THE   HARDENING   OF   STEEL 


form  speed  but  so  regulated  that  the  transformation  of  austenite  proceeds  only  to 
the  desired  extent,  to  wit,  cooling  in  water  for  martensite,  in  oil  for  sorbite. 

An  attempt  has  been  made  in  Figure  13  to  give  a  graphical  illustration  of  the 
cooling  conditions  needed  for  the  production  of  the  various  constituents  of  steel. 
Its  interpretation  will  be  obvious.  The  critical  range,  or  rather  the  lower  critical 
point,  Ari  or  Ar3.2.i  is  represented  as  covering  a  considerable  range  of  temperature 
so  as  to  afford  the  necessary  room  for  the  diagrammatical  representation  of  the  for- 
mation, within  that  range,  of  the  transition  constituents.  The  diagram  indicates  that 
as  the  metal  cools  slowly  through  its  range  it  does  not  pass  abruptly  from  an  austenitic 
to  a  martensitic  condition  and  then  to  troostite,  etc.,  but  that  these  transformations 


X 
P 


/  /  / 

s  T  M  A 


Avsfenite 


A    M   T 


A  =  Austenite 
M  =  Martensite 
T  =  Troostite 
S  =  Sorbite 
P  =Pearlite 

Fig.  13.  —  Diagram  depicting  the  formation  of  austenite,  martensite,  troostite,  sorbite,  and  pearlite 

in  steel  cooling  through  its  critical  range. 

are,  on  the  contrary,  gradual,  the  following  types  of  structure  being  formed,  theoreti- 
cally at  least :  austenite,  austenite  plus  martensite,  martensite,  martensite  plus  troost- 
ite, troostite,  troostite  plus  sorbite,  sorbite,  sorbite  plus  pearlite,  and  pearlite.  The 
transformations  depicted  refer  to  eutectoid  steel  or  to  the  residual  austenite  (neces- 
sarily of  eutectoid  composition)  of  hypo-  and  hyper-eutectoid  steel  formed  on  slow 
cooling  to  Ari  after  rejection  of  free  ferrite  or  free  cementite.  In  the  cases  of  these 
steels,  therefore,  free  ferrite  or  free  cementite  is  present  in  the  above  structures, 
unless,  indeed,  cooling  between  Ar3  and  Ari  or  between  Arcm  and  Ari  has  been  so  rapid 
as  to  prevent  their  separation.  While,  theoretically,  very  quick  cooling  from  a  to  at- 
mospheric temperature  should  retain  the  steel  in  its  austenitic  condition,  even  under 
the  most  favorable  conditions,  this  can  be  done  but  partially,  considerable  martensite 
being  produced.  Cooling  slowly  to  m  and  then  quickly  should  produce  martensite 


LESSON  XIII  — THE  HARDENING  OF  STEEL  17 

while  slow  cooling  to  t  or  s  followed  by  quick  cooling  should  produce,  respectively, 
troostite  and  sorbite.  Slow  cooling  to  p,  followed  or  not  by  quick  cooling,  results  in 
the  formation  of  pearlite.  Slow  cooling  to  intermediate  points  between  m  and  t  or 
t  and  s,  etc.,  should,  theoretically,  cause  the  formation  of  martensite  and  troostite, 
troostite  and  sorbite,  etc. 

These  conditions  may  be  realized  in  the  same  piece  of  steel  by  heating  one  end  of 
a  steel  bar,  preferably  of  eutectoid  or  hyper-eutectoid  composition,  well  above  the 
critical  range  and  quenching  the  whole  bar  in  ice-cold  water.  It  is  evident  that,  since 
at  the  time  of  quenching  the  temperature  of  the  bar  decreased  gradually  from  the 
hot  to  the  cold  end,  a  portion  of  the  bar  must  have  been  quenched  while  in  the  mar- 
tensitic  condition,  another  while  in  the  troostitic  condition,  etc.  The  preparation 
and  microscopical  examination  of  longitudinal  sections  through  the  center  of  the  bar 
should  reveal  the  existence  of  the  various  constituents  indicating  as  many  stages  in 
the  transformation  of  austenite. 

On  the  left  of  the  diagram  five  lines  starting  from  the  point  R  above  the  criti- 
cal range  represent  coolings  through  the  range  at  different  speeds  as,  for  instance, 
(1)  very  quickly  in  ice-cold  water,  (2)  quickly  in  water,  (3)  less  quickly  in  oil,  (4) 
slowly  in  air,  and  (5)  very  slowly  in  furnace,  resulting,  respectively,  in  the  forma- 
tion of  austenite  (or  rather  austenite  and  martensite),  martensite,  troostite,  sor- 
bite, and  pearlite  or,  more  frequently,  of  mixtures  of  two  or  even  three  of  these 
constituents. 

These  conditions  may  be  realized  in  the  same  piece  of  metal  by  heating  a  steel 
bar  of  considerable  cross  section  (not  less  than  one  inch  in  diameter)  and  preferably 
of  eutectoid  or  hyper-eutectoid  composition  to  a  temperature  well  above  the  critical 
range  and  quenching  it  in  water.  The  cooling  should  be  rapid  enough  to  cause  the 
formation  of  martensite  near  the  outside  of  the  piece  while  the  cooling  of  the  center 
should  be  so  slow  (because  of  the  size  of  the  bar)  as  to  permit  the  formation  of  pear- 
lite.  Between  the  martensitic  shell  and  the  pearlitic  core  the  metal  should  be  com- 
posed of  the  other  transition  constituents,  that  is,  starting  from  the  center,  of  sorbite 
and  then  of  troostite.  The  microscopical  examination  of  a  cross  section  through  the 
bar  should  reveal  this  gradual  change  of  structure  from  center  to  outside. 

The  formation  of  a  transition  constituent  through  the  tempering  of  a  constituent 
representing  a  less  advanced,  and  therefore  less  stable,  stage  of  transformation  has 
already  been  alluded  to  and  will  be  considered  at  greater  length  in  the  next  lesson. 

Are  the  Transition  Stages  Distinct  Constituents?  —  It  would  appear  from  our  con- 
sideration of  the  formation  of  the  transition  constituents  that  they  must  represent 
as  many  stages  in  the  progressive  transformation  of  austenite  and  that  sharp  lines  of 
demarcation  between  them  are  not  to  be  expected  or,  rather,  that  they  must  be  linked 
together  by  an  unbroken  chain  of  transition  stages  just  as  the  blue  and  yellow  colors 
are  connected  through  an  unbroken  series  of  bluish,  greenish,  and  yellowish  shades. 
This  logical  inference,  however,  is  not  supported  by  microscopical  evidences  in  the 
cases  of  austenite,  martensite,  and  troostite  for  these  constituents  are  sharply  differ- 
entiated from  each  other  whenever  they  occur  together.  Stages  or  structural  condi- 
tions representing  the  gradual  transformation  of  austenite  into  martensite  or  of 
martensite  into  troostite  are  not  observed;  it  is  as  if  these  transformations  had  actu- 
ally taken  place  by  rather  sudden  steps. 

The  transformations  of  troostite  into  sorbite  and  of  sorbite  into  pearlite,  on  the 
contrary,  appear  to  be  very  gradual  and  easy  to  follow.  In  other  words  while  troost- 
ite, sorbite,  and  pearlite  are  readily  distinguishable,  each  having  marked  charac- 


18  LESSON  XIII  — THE   HARDENING   OF  STEEL 

teristics  of  its  own,  structural  arrangements  are  frequently  observed  which  undoubtedly 
correspond  to  intermediate  stages. 

The  five  constituents  of  steel,  resulting  from  various  modes  and  rates  of  cooling', 
might  be  compared  to  a  spectrum  of  five  elementary  colors,  i.e.  violet,  blue,  yellow, 
orange,  and  red,  with  gaps  existing  between  the  first  three  (our  austenite,  martensite, 
and  troostite)  while  the  third  and  the  fifth,  yellow  and  red  (troostite  and  pearlite) 
are  closely  linked  together  by  the  fourth,  orange  (our  sorbite),  and  an  infinity  of  in- 
termediate shades. 

Metarals  and  Aggregates.  —  Howe  suggests  dividing  all  microscopical  constit- 
uents of  iron  and  steel  into  (1)  metarals  and  (2)  aggregates. 

These  he  describes  as  follows:  "Metarals,  true  phases  like  the  minerals  of  nature. 
These  are  like  definite  chemical  compounds,  or  solid  solutions,  and  hence  consisting 
of  definite  substances  in  varying  proportions  .  .  .  Aggregates,  like  the  petrographic 
entities  as  distinguished  from  the  true  minerals.  These  mixtures  may  be  in  definite 
proportions,  i.e.  eutectic  or  eutectoid  mixtures  (ledeburite,  pearlite,  steadite)  or  in 
indefinite  proportions  (troostite,  sorbite)."  1  Under  these  two  headings  the  constit- 
uents of  iron-carbon  alloys  would  be  classified  as  follows: 

Metarals:  ferrite,  cementite,  austenite,  graphite. 

Aggregates:  pearlite,  sorbite,  troostite,  ledeburite,  steadite. 

Opinions  differ  as  to  the  nature  of  martensite;  if  it  is  a  solid  solution  it  is  a  metaral, 
if  not  it  must  be  classified  with  the  aggregates.  Should  we  recognize  the  existence 
of  solid  colloidal  solutions,  it  is  not  clear  whether  these  should  be  grouped  with  the 
metarals  or  should  form  a  distinct  class  between  the  metarals  and  the  aggregates. 

Hardening  Eutectoid  Steel.  —  It  will  now  be  profitable  to  consider  separately 
the  hardening  of  eutectoid,  hyper-eutectoid,  and  hypo-eutectoid  steel.  In  hardening 
eutectoid  steel  the  metal  should  be  heated  through  its  critical  range,  i.e.  through  its 
single  critical  point  Ac3.2.i.  By  so  doing  we  confer  upon  it  full  hardening  power  and 
finest  possible  structure.  The  steel  should  then  be  cooled  from  that  temperature  as 
promptly  as  possible  avoiding  heating  much  above  the  range  or  long  exposure  at  any 
temperature  above  the  range,  as  either  procedure  would  tend  to  increase  the  grain 
size  of  the  metal.  A  temperature  of  some  775  to  825  deg.  C.  will  generally  be,  there- 
fore, the  best  temperature  to  which  to  heat  and  from  which  to  cool  eutectoid  steel  for 
the  purpose  of  hardening.  By  this  treatment  the  steel  passes  from  a  finely  austenitic 
to  a  finely  martensitic  or  troosto-martensitic  condition  (Fig.  8). 

Hardening  Hyper-Eutectoid  Steel.  —  Let  us  assume  a  steel  containing  1.25  per 
cent  carbon  and,  therefore,  composed  approximately  of  93  per  cent  of  pearlite  and 
7  per  cent  of  free  cementite.  Upon  heating  this  steel  through  its  lower  point  Ac3.2.i  its 
93  per  cent  of  pearlite  are  converted  into  93  per  cent  of  austenite  possessing  hardening 
power,  but  the  metal  still  contains  7  per  cent  of  free  cementite  deprived  of  hardening 
power.  If  we  heat  it  past  its  upper  point  Accm  the  free  cementite  is  absorbed  and  the 
whole  mass  becomes  austenitic.  A  little  reflection  will  show,  however,  that  the  steel 
should  be  quenched  as  soon  as  it  rises  above  the  point  Ac3.2.i,  because  we  then  pro- 
duce, theoretically  at  least,  93  per  cent  of  fine  grained  martensite  while  retaining,  to 
be  sure,  the  original  7  per  cent  of  cementite,  but  as  this  constituent  is  harder  than 

1  Howe  further  writes:  "Many  true  minerals,  such  as  mica,  felspar,  and  hornblende,  are  divisible 
into  several  different  species.  Such  minerals  are  definite  chemical  compounds,  in  which  one  element 
may  replace  another.  Others,  such  as  obsidian,  are  solid  solutions  in  varying  proportions  and  in  these 
also  one  element  may  replace  another.  Metarals  like  minerals  differ  from  aggregates  in  being  sever- 
ally chemically  homogeneous." 


LESSON   XIII  — THE   HARDENING   OF   STEEL  19 

martensite  its  presence  adds  to,  rather  than  takes  away  from,  the  hardness  of  the 
quenched  metal.  Should  we,  on  the  contrary,  heat  to  above  Accm  before  quenching 
the  whole  mass  would  be  converted  into  martensite  but  it  would  be  less  hard  if  any- 
thing than  the  metal  quenched  at  a  lower  temperature  while  its  structure  would  be 
coarser  and  the  danger  of  cracking  the  objects  in  the  quenching  bath  would  be  greater. 
The  best  hardening  temperature  for  hyper-eutectoid  steels,  therefore,  is  the  same  as 
that  for  eutectoid  steel,  namely,  some  775  to  825  deg.  C. 

The  structure  of  a  properly  hardened  hyper-eutectoid  steel  is  shown  in  Figure  14. 
Like  hardened  eutectoid  steel  it  consists  of  very  fine  martensite. 

Hardening  Hypo-Eutectoid  Steel.  —  Let  us  take  a  steel  containing  some  0.50  per 
cent  carbon  and  exhibiting  therefore  the  points  AI  and  A3.2.  ~S'urh  steel  is  made  up 


Fig.  14.  —  Steel.    Carbon  1.10  per  cent.    Magnified  100  diameters.    Quenched 
in  water  from  above  its  critical  range.     (Boylston.) 

of  60  per  cent  of  pearlite  and  40  per  cent  free  ferrite.  Upon  heating  it  through  its 
Aci  point  the  pearlite  is  converted  into  austenite,  so  that  at  this  temperature  some 
60  per  cent  of  the  mass  of  the  metal  is  endowed  with  hardening  power.  Should  we 
quench  this  steel,  therefore,  as  soon  as  it  rises  above  its  lower  critical  point  but  60 
per  cent  of  its  bulk  would  be  hardened ;  it  would  still  retain  40  per  cent  of  soft  ferrite. 
.If,  on  the  contrary,  the  heating  be  carried  to  just  above  Ac3.2  the  free  ferrite  is  absorbed 
by  the  austenite  and  the  whole  mass  becomes  harrienable.  Upon  quenching  the  steel 
from  that  temperature  its  entire  bulk  may  be  converted  into  martensite  or  troosto- 
martensite,  according  to  rate  of  cooling.  While  this  martensite  will  not  be  quite  as 
hard  as  the  martensite  produced  by  quenching  from  just  above  Aci  the  metal  as  a 
whole  will  be  harder  and  of  a  more  uniform  and  finer  structure  because  of  the  absence 
of  free  ferrite,  or  at  least  of  any  considerable  amount  of  it.  It  follows  from  these  con- 
siderations that  for  the  purpose  of  hardening,  hypo-eutectoid  steel  should  be  quenched 
from  just  above  its  upper  critical  point,  namely,  Ac3.2  or  Ac3  (825  to  925  deg.  C.  ac- 
cording to  carbon  content).  Hypo-eutectoid  steel  containing  very  little  carbon,  say 


20  LESSON  XIII  — THE  HARDENING  OF  STEEL 

less  than  0.25  per  cent,  cannot  be  very  materially  hardened  by  the  ordinary  quench- 
ing methods  because  of  the  large  amount  of  soft  ferrite  which  it  contains  in  excess  of 
the  eutectoid  ratio  and  which  cannot  be  retained  in  solution,  even  on  very  quick 
cooling  (see  Lesson  XII,  Fig.  11).  The  structure  of  hardened  hypo-eutectoid  steel 
is  shown  in  Figure  1. 

Steel  of  Maximum  Hardening  Power.  —  From  the  above  considerations  it  will  be 
obvious  that  the  hardening  of  steel  consists  in  preventing  the  formation  of  relatively 
soft  pearlite  and  in  causing,  instead,  the  formation  and  retention  of  hard  martensite 
or  troostite  or,  more  often,  of  both.  It  follows  from  this  that  the  steel  possessing 
maximum  hardening  power  must  be  that  steel  which  in  slow  cooling  would  contain 
most  pearlite,  namely,  eutectoid  steel.  It  does  not,  of  course,  mean  that  quenched 
eutectoid  steel  is  harder  than  quenched  hyper-eutectoid  steel  but  merely  that  the  in- 
creased hardness  produced  by  quenching  is  greatest  in  the  case  of  eutectoid  steel. 
Quenched  hyper-eutectoid  steel  is  harder  than  quenched  eutectoid  steel  because  of 
the  presence  in  the  former  of  some  free  cementite  or  of  more  highly  carburized  mar- 
tensite but  the  difference  of  hardness  between  the  two  steels  is  greater  before  quenching. 

Hardening  Large  Pieces.  —  In  hardening  pieces  of  considerable  cross  section  it  is 
evident  that  the  central  portions  will  not  cool  as  quickly  as  the  outside  and  will  not, 
therefore,  be  as  hard.  Indeed  the  center  may  cool  so  slowly  that  it  will  fail  to  harden 
at  all.  The  limitation  of  the  hardening  process,  as  applied  to  large  pieces,  will,  there- 
fore, be  evident.  It  is  seldom  desirable,  however,  to  harden  large  pieces  to  their 
very  core.  When  large  steel  objects  are  to  be  hardened,  as  for  instance  in  the  case  of 
armor  plates,  superficial  hardness  only  is  desired,  or  at  least  hardness  penetrating  to 
but  a  relatively  small  depth,  and  this  is  readily  secured  through  the  case  hardening 
process. 

Hardening  and  Tempering  in  One  Operation.  —  A  peculiar  but  frequent  way  of 
hardening  tools  consists  in  heating  the  tool  to  the  proper  temperature  and  then  cool- 
ing quickly  to  a  black  heat  only  that  portion  of  it  which  should  be  hard,  while  keep- 
ing the  other  portion  out  of  the  bath  and,  therefore,  at  a  high  temperature.  Upon 
withdrawing  the  tool  from  the  bath  the  heat  stored  away  in  the  hot  portion  diffuses 
to  the  cooled  portion  which  is  in  this  way  reheated,  that  is,  tempered,  as  later  ex- 
plained, the  amount  of  tempering  being  regulated  at  will  by  again  quenching  the 
metal  as  soon  as  the  desired  tempering  color  has  been  obtained. 

Experiments 

The  following  experiments  will  prove  instructive.  A  steel  bar  about  one  half  inch 
square  or  round  and  containing  some  0.50  per  cent  carbon  should  be  heated  at  one 
end,  conveniently  in  a  forge,  in  such  a  way  that  the  extreme  end  will  be  at  a  very 
bright  red  or  yellow  heat  while  its  color  three  inches  from  the  end  should  not  be  more 
than  a  very  dull  red.  The  whole  bar  should  then  be  quenched  in  cold  water.  Three 
pieces  one  inch  long  should  be  detached  from  the  heated  end  of  the  bar  and  a 
longitudinal  section  of  each  piece  polished,  etched,  and  microscopically  examined. 
Starting  with  the  piece  that  was  heated  to  the  highest  temperature  the  constituents 
of  hardened  steel  should  be  noted  in  the  ordinary  order,  i.e.  martensite,  troostite, 
sorbite,  and  in  the  unhardened  portion,  pearlite,  or  association  of  two  or  more  of 
these.  Free  ferrite  will  occur,  of  course,  in  the  pearlitic  portion  while  it  may  also  be 
observed,  but  in  smaller  quantity,  in  the  sorbitic  and  even  troostitic  zones. 

Small  pieces  of  hypo-eutectoid,  eutectoid,  and  hyper-eutectoid  steels  should  be 


LESSON   XIII  — THE   HARDENING   OF  STEEL  21 

heated  above  their  respective  critical  range  and  quenched  in  cold  water  and  some  in 
oil.  Cross  sections  of  all  pieces  should  be  prepared  for  examination  which  should  show 
that  the  pieces  quenched  in  water  are  martensitic  or  troosto-martensitic  while  those 
quenched  in  oil  are  chiefly  troostitic  or  troosto-sorbitic.  In  hypo-eutectoid  steel  free 
ferrite  and  in  hyper-eutectoid  steel  free  cementite  are  likely  to  occur. 

If  a  sample  of  steel  can  be  obtained  containing  not  less  than  1.50  per  cent  carbon 
and  not  less  than  1  per  cent  manganese  (preferably  2  per  cent)  a  small  piece  of  this 
steel  should  be  heated  to  1100  deg.  C.,  or  higher,  and  quenched  in  ice-cold  water. 
This  treatment  should  produce  a  martenso-austenitic  structure. 

Etching.  —  The  reagents  already  described  for  etching  pearlitic  and  sorbitic  steels, 
namely,  solutions  of  nitric  and  picric  acid  in  alcohol,  may  be  employed  with  generally 
satisfactory  results  for  the  etching  of  hardened  steels.  As  these  etch  much  more 
quickly,  however,  the  immersions  should  be  correspondingly  shorter,  especially  when 
the  metal  contains  troostite.  Picric  acid  being  slower  in  its  action  than  nitric  acid  is 
preferred  by  some  for  etching  hardened  steel.  The  Kourbatoff  reagent  may  be 
tried. 

Examination 
I.  —  Describe  the  treatment  necessary  to  impart  hardening  power  to  steel. 

II.  —  Explain  the  influence  of  the  rate  of  cooling  through  the  critical  range  on  the 
structure  of  steel  and  describe  briefly  the  constituents  resulting  from  cooling 
through  that  range  at  various  speeds. 


LESSON  XIV 

THE  TEMPERING  OF  HARDENED  STEEL 

Steel  that  has  been  hardened  by  rapid  cooling  from  above  its  critical  range,  as  ex- 
plained in  the  preceding  lesson,  is  often  harder  than  necessary  and  generally  too  brittle 
for  most  purposes.  In  order  to  decrease  its  brittleness,  that  is,  to  toughen  it  without 
very  material  diminution  of  hardness,  the  metal  is  generally  "  tempered,"  that  is,  re- 
heated to  a  temperature  considerably  below  its  critical  range.  This  operation  is  called 
tempering  because  it  somewhat  mitigates  or  tempers  the  effects  of  the  previous  harden- 
ing treatment. 

Tempering  Temperatures.  —  The  hardening  of  steel  causes  increased  hardness, 
brittleness,  and  elastic  limit,  all  of  which  are  somewhat  lowered  by  the  tempering 
operation.  The  effect  of  tempering  begins  to  be  noticeable  at  about  100  deg.  C. 
and  increases  in  intensity  as  the  temperature  rises,  until  finally  at  some  600  deg.  the 
metal  assumes  again  the  physical  properties  characteristic  of  the  unhardened  con- 
dition. The  temperature  to  which  hardened  steel  should  be  heated  for  tempering 
varies,  therefore,  with  the  use  to  which  it  is  destined.  If  it  is  desired  to  retain  the 
greatest  possible  hardness,  necessarily  with  its  accompanying  brittleness,  the  steel 
should  be  reheated  but  slightly  above  200  deg.  C.,  as,  for  instance,  in  tempering  razor 
blades  when  extreme  hardness  is  essential  and  brittleness  relatively  immaterial.  If, 
on  the  contrary,  considerable  toughness  is  indispensable,  at  the  necessary  sacrifice  of 
some  hardness  the  steel  should  be  tempered  to  some  300  deg.  C.  or  even  to  a  higher 
temperature.  The  great  majority  of  tools  are  tempered  between  200  and  300  deg. 

Tempering  Colors.  —  Hardened  steel  objects  subjected  to  tempering  being  gen- 
erally quite  bright  and  their  heating  being  generally  conducted  in  an  oxidizing  atmos- 
phere, very  thin  films  of  oxides  form  upon  their  surfaces.  The  colors  of  these  films 
vary  with  the  temperature,  that  is,  to  each  tempering  temperature  corresponds  a  cer- 
tain color,  and  blacksmiths  generally  depend  upon  these  colors  for  the  tempering  of 
their  tools,  the  use  of  pyrometers  for  this  operation  being  far  from  general.  Accord- 
ing to  Howe  the  tempering  colors  and  corresponding  temperatures  are  as  follows: 

Pale  yellow 220  deg.  C.  or  428  deg.  F. 

Straw 230  "     "    "  446  "" 

Golden  yellow 243  """  469  "" 

Brown      255  "     "    "  491  "" 

Brown  dappled  with  purple 265  "     "    "  509  "     " 

Purple      277  """  531  "     " 

Bright  blue 288  "     "    "  550  "" 

Pale  blue 297  "     "    "  567  "" 

Dark  blue 316  "      "    "  600  "     " 

In  order  that  the  tempering  colors  may  be  plainly  seen  the  steel  objects  should  be 
smooth  and  bright,  preferably  polished. 

Time  at  Tempering  Temperature.  —  It  is  the  common  belief  that  once  the  desired 
temperature  is  obtained,  as  indicated  by  the  color,  little  is  to  be  gained  by  maintaining 

1 


2  LESSON   XIV  — THE   TEMPERING   OF   HARDENED   STEEL 

the  steel  at  that  temperature  any  length  of  time  on  the  ground  that  it  will  not  result 
in  producing  additional  tempering.  The  tempering  of  steel  has  been  compared  to  the 
releasing  of  a  spring  permitting  a  certain  structural  rearrangement,  that  is,  a  certain 
tempering  of  the  metal  at  any  temperature  but  not  more.  To  produce  additional 
tempering  the  temperature  must  be  increased,  that  is,  the  spring  must  be  further 
released.  Recent  investigations,  however,  have  shown  that  the  maintenance  of  hard- 
ened steel  at  a  certain  tempering  temperature  often  does  produce  additional  temper- 
ing effect.  It  was  further  ascertained  that  the  color,  instead  of  remaining  unchanged 
at  any  given  temperature,  advances  in  the  tempering  color  scale  as  it  would  with  in- 
creasing temperature.  In  other  words,  the  tempering  colors,  contrary  to  the  view 
generally  held,  are  not  an  absolute  criterion  by  which  to  judge  of  the  temperature  of 
the  steel,  since  they  vary  with  the  length  of  time  during  which  the  steel  is  kept  at  any 
temperature.  These  experiments  seem  to  show,  however,  that  the  amount  of  temper- 
ing effected  is  closely  related  to  the  color,  that  is,  that  to  each  shade  corresponds  a 
certain  amount  of  tempering.  It  should,  however,  be  borne  in  mind  that  these  colors, 
with  the  corresponding  tempering  they  imply,  may  be  obtained  in  two  ways,  (1)  through 
short  exposure  at  a  certain  temperature  and  (2)  through  longer  exposure  at  lower 
temperatures.  The  same  amount  of  tempering,  for  instance,  would  result  (a)  from 
heating  hardened  steel  to  288  deg.,  when  its  color  is  bright  blue,  followed  by  immediate 
cooling  and  (6)  from  heating  it  to  255  deg.,  when  its  color  is  brown,  and  maintaining  it 
at  that  temperature  until  its  color  becomes  bright  blue.  Unless  baths  kept  at  con- 
stant temperatures  are  used,  however,  it  is  evident  that  in  practise  the  steel  should 
be  quenched  as  soon  as  the  desired  color  is  produced  and  while  its  temperature  is 
rising,  because  it  is  simpler  and  more  convenient  to  produce  that  color  on  a  rising 
temperature  than  by  maintaining  the  metal  at  a  constant  temperature. 

Some  writers  doubt  the  existence  of  so  close  a  relation  between  the  color  and  the 
resulting  tempering.  According  to  Barus  and  Strouhal  to  each  tempering  tempera- 
ture corresponds  a  maximum  tempering  effect  which  is  the  more  quickly  reached  the 
higher  the  temperature.  At  100  deg.,  for  instance,  the  maximum  effect  was  not  ob- 
tained after  one  hour  although  maintaining  the  steel  at  that  temperature  two  more 
hours  had  but  little  additional  effect.  At  200  deg.  the  maximum  effect  was  obtained 
in  ten  minutes,  while  at  300  deg.  one  minute  was  sufficient. 

Rate  of  Cooling  from  Tempering  Temperature.  —  Once  the  desired  amount  of 
tempering  is  effected,  as  indicated  by  the  color  or  otherwise,  the  rate  of  cooling  to  at- 
mospheric temperature  appears  to  be  quite  immaterial.  In  practise  the  piece  is  gener- 
ally quenched,  merely  for  convenience.  The  theory  is  that  while  by  keeping  the 
metal  at  a  certain  temperature  its  tempering  may  be  carried  farther,  on  cooling 
tempering  ceases,  for  the  spring  is  now  tightened,  to  use  the  simile  already  referred 
to,  so  that  the  rate  of  cooling  is  without  influence. 

Hardening  and  Tempering  Combined.  —  A  method  of  hardening  and  tempering 
combined,  frequently  employed  when  but  one  end  of  a  tool  must  be  hardened  as  in 
the  case  of  chisels  and  drills,  has  been  described  in  the  preceding  lesson.  It  consists  in 
heating  the  tool  above  its  critical  range,  quenching  that  portion  only  which  is  to  be 
hard,  removing  it  from  the  bath,  and  allowing  the  heat  stored  in  the  unquenched  por- 
tion to  heat  by  conduction  the  quenched  part  until  the  desired  temper  color  is  ob- 
tained, when  it  is  again  quenched  lest  the  tempering  be  carried  too  far. 

Explanation  of  the  Tempering  of  Steel.  —  The  theories  accounting  for  the  temper- 
ing of  steel  will  be  considered  in  the  next  lesson  together  with  the  hardening  theories. 
It  will  suffice  for  the  present  to  point  out  that  hardened  steel  is  generally  considered 


LESSON  XIV  — THE   TEMPERING   OF   HARDENED   STEEL  3 

to  be  in  an  unstable  condition  and,  therefore,  eager  to  return  to  a  more  stable  form  and 
actually  undergoing  this  change  whenever  given  an  opportunity,  that  is  on  raising 
its  temperature.  At  atmospheric  temperature  the  passage  of  an  unstable  martensitic 
condition  into  a  more  stable  troostitic  or  sorbitic  form  is  prevented  by  the  rigidity  of 
the  metal.  A  slight  heating,  however,  produces  some  plasticity  and  such  transforma- 
tion takes  place  to  a  small  extent.  On  increasing  the  temperature  the  rigidity  dimin- 
ishes farther  and  the  transformation  advances.  The  tempering  of  hardened  steel,  in 
other  words,  is  due  to  its  transformation  from  an  unstable  condition  to  one  more 
stable.  It  will  be  seen  that  at  some  600  deg.  C.  the  metal  assumes  a  stable  condition, 
i.e.  is  fully  tempered. 

Tempering  Austenitic  Steels.  —  It  has  been  explained  in  the  preceding  lesson  that 
austenite,  martensite,  troostite,  and  even  sorbite  were  the  constituents  formed  in 
hardened  steel  according  to  the  rapidity  with  which  the  metal  is  cooled  through 
and  below  its  critical  range.  It  will  now  be  instructive  to  consider  separately  the 
tempering  of  steels  having  these  different  types  of  structure. 

Austenitic  carbon  steel,  as  already  stated,  is  not  a  commercial  article,  as  it  requires 
for  its  production,  at  least  in  the  absence  of  a  large  proportion  of  manganese,  the 
presence  of  much  carbon,  an  excessively  high  quenching  temperature,  and  a  quench- 
ing bath  at  a  very  low  temperature.  And  even  when  these  conditions  prevail,  only 
one  half  or  so  of  the  bulk  of  the  steel  can  be  retained  in  an  austenitic  condition,  the 
other  half  being  martensitic.  The  tempering  of  austenite  should,  nevertheless,  be  con- 
sidered. Since  in  austenitic  steel  the  condition  of  the  metal  stable  only  above  the 
critical  range  has  been  retained  in  the  cold,  it  follows  that  cold  austenitic  steel  must 
be  in  a  very  unstable  condition.  At  atmospheric  temperature  the  rigidity  of  the 
metal  is  so  great  that  a  return  to  a  more  stable  form  is  not  possible  but,  on  heating  it 
very  slightly,  sufficient  plasticity  is  produced  to  permit  a  partial  transformation  of 
austenite.  This  partial  transformation,  theoretically  at  least,  should  result  in  the 
formation  of  martensite,  troostite,  and  sorbite  in  the  order  named  as  the  tempering 
temperature  increases.  This  has  been  depicted  in  I,  Figure  1.  In  this  diagram  it  is 
shown  (1)  that  as  soon  as  austenite  is  heated  above  atmospheric  temperature  it  begins 
to  be  converted  into  martensite,  (2)  that  it  is  entirely  converted  into  martensite  at 
200  deg.  C.,  (3)  that  in  heating  above  200  deg.  martensite  begins  to  pass  to  troostite, 
(4)  that  at  400  deg.  the  transformation  of  martensite  into  troostite  is  complete,  (5)  that 
above  400  deg.  troostite  is  gradually  converted  into  sorbite,  (6)  that  at  600  deg.  C. 
the  transformation  of  troostite  into  sorbite  is  complete,  and  (7)  that  the  sorbite  con- 
dition is  the  final  condition  acquired  by  hardened  steel  when  reheated  close  to,  but 
below,  its  critical  range.  While  it  is  certain  that  lamellar,  i.e.  true  pearlite,  cannot  be 
formed  by  reheating  hardened  steel,  it  is  claimed  by  some  that  long  heating  of  sorbite 
near  the  critical  range,  that  is,  between  600  and  700  deg.  C.,  will  result  in  the  forma- 
tion of  granular  pearlite  brought  about  as  explained  in  another  lesson  by  the  spheroi- 
dizing  of  the  cementite.  The  tempering  of  austenite  depicted  in  I,  Figure  1,  represents 
the  transformation  which,  according  to  theoretical  consideration,  should  be  expected 
to  take  place.  Most  observers,  however,  report  that  on  tempering  austenite  it  is  at  once 
converted  into  troostite  as  soon  as  the  rigidity  of  the  steel  has  been  sufficiently  relaxed, 
the  martensitic  stage  not  being  assumed.  This  is  shown  diagrammatically  in  II, 
Figure  1.  Here,  again,  it  is  indicated  that  at  400  deg.  the  steel  is  entirely  troostitic 
and  at  600  deg.  entirely  sorbitic.  Benedicks  thinks  that  it  is  quite  possible  that  aus- 
tenite is  first  transformed  into  martensite  but  that  the  resulting  martensite  is  so 
readily  and  quickly  converted  into  troostite  that  its  short  existence  easily  escapes 


LESSON  XIV  — THE  TEMPERING  OF  HARDENED  STEEL 


*      I 


I 


t 
I 

I 


LESSON  XIV  —  THE  TEMPERING   OF  HARDENED  STEEL  5 

observation.  His  belief  is  based  upon  the  following  considerations:  (1)  the  austenite 
retained  in  high  carbon  steel  by  very  sudden  cooling  is  subjected  to  great  pressure 
caused  by  the  accompanying  martensite  having  been  formed  with  considerable  dilata- 
tion, (2)  on  reheating  this  martenso-austenitic  steel  the  martensite  is  first  converted 
into  troostite,  because  this  transformation  taking  place  with  contraction  is  readily 
induced,  (3)  once  this  transformation  started,  the  pressure  upon  the  austenite  is  re- 
leased and  this  constituent,  in  turn,  passes  first  to  the  martensitic  stage  (with  increase 
of  volume)  and  then  quickly  to  the  troostitic  stage  (with  decrease  of  volume)  the 
martensitic  condition  being  of  so  short  duration  as  to  readily  escape  detection. 

Tempering  Martensitic  Steel.  —  It  has  been  shown  that  martensite  is  generally 
present  in  commercially  hardened  steel.  Since  this  constituent  represents  a  partial 
transformation  of  austenite  it  follows  that  it  must  be  more  stable  than  austenite  at 
atmospheric  temperature.  It  is  sufficiently  unstable,  however,  to  be  readily  con- 
verted first  into  troostite  and  then  into  sorbite  on  tempering  as  indicated  in  III, 
Figure  1.  At  400  deg.  the  transformation  of  martensite  into  troostite  is  complete, 
while  at  000  deg.  troostite  is  replaced  by  sorbite.  Bearing  in  mind  the  physical  prop- 
erties of  martensite,  troostite,  and  sorbite,  it  will  be  readily  understood  why,  on  temper- 
ing martensitic  steel,  its  hardness  gradually  decreases  while  it  becomes  less  brittle  and 
indeed  quite  ductile  if  made  sorbitic. 

Tempering  Troostitic  Steel.  —  Commercially  hardened  steel  frequently  contains 
large  proportions  of  troostite.  The  tempering  of  troostite  is  depicted  in  IV,  Figure  1. 
This  constituent  1  icing  decidedly  less  unstable  than  martensite  requires  greater  plastic- 
ity, i.e.  a  higher  temperature,  before  being  transformed  into  a  still  more  stable  condi- 
tion. Experimental  evidences  seem  to  show  that  the  tempering  of  troostite,  i.e.  its 
transformation  into  sorbite,  requires  a  temperature  of  at  least  400  deg.  and  that  at 
600  deg.  the  transformation  is  complete.  Since  in  practise  the  tempering  of  steel  is 
seldom  carried  above  300  deg.  it  would  seem  as  if  steels  made  up  of  troostite  do  not 
need  to  be  tempered,  being  sufficiently  tough.  There  seems  to  be  no  reason  to  doubt 
the  accuracy  of  the  above  inference.  Commercially  hardened  steels,  however,  are 
generally  either  entirely  martensitic  or,  more  frequently,  partly  martensitic  and  partly 
troostitic  and  are  in  need  of  tempering  because  of  the  large  proportion  of  the  exces- 
sively hard  and  brittle  martensite  they  usually  contain. 

Tempering  Troosto-Martensitic  Steel.  —  In  V,  Figure  1,  the  tempering  of  hard- 
ened steel  containing  both  martensite  and  troostite  has  been  depicted.  It  is  assumed 
that  the  martensite  present  begins  to  be  converted  into  troostite  as  soon  as  the  tem- 
perature of  the  metal  rises,  while  the  transformation  of  the  troostite  into  sorbite  begins 
only  at  400  deg. 

Tempering  Troosto-Sorbitic  Steel.  —  From  the  diagram  used  to  illustrate  the 
tempering  of  steel  it  will  be  apparent  that  sorbite  is  relatively  so  stable  a  constituent 
that  its  transformation  into  pearlite  cannot  be  effected  below  the  critical  range  or,  in 
other  words,  that  it  cannot  be  tempered.  Indeed  sorbitic  steels  not  being  hardened 
steels  need  not  be  considered  in  connection  with  the  tempering  operation.  A  graphi- 
cal representation  of  the  tempering  of  troosto-sorbitic  steel,  however,  has  been 
included  in  Figure  1 .  It  shows  that  such  steel  remains  unchanged  until  its  tempera- 
ture reaches  400  deg.  when  the  troostite  it  contains  begins  to  be  transformed  into 
sorbite,  the  transformation  being,  as  usual,  complete  at  600  deg. 

Osmondite.  —  It  has  been  shown  that  on  tempering  hardened  steel  it  is  entirely 
converted  into  troostite  at  about  400  deg.  C.  Below  that  temperature  some  marten- 
site  remains  in  the  structure,  while  above  it  some  sorbite  is  present.  To  the  condition 


6 


LESSON   XIV  — THE   TEMPERING   OF   HARDENED   STEEL 


of  steel  when  made  up  wholly  of  troostite  Heyn  gives  the  name  of  "osmondite."  It 
will  he  apparent  that  osmondite  does  not  represent  a  new  constituent  but  merely  a 
condition  assumed  by  the  steel  at  a  certain  temperature  and  the  wisdom  of  giving  a 
specific  name  to  that  condition  may  well  be  questioned. 

Troostite  is  more  readily  colored  by  the  usual  etching  reagents  than  any  other 
constituent  of  steel,  from  which  it  follows  that  steel  made  up  exclusively  of  troostite, 
i.e.  in  the  osmondite  condition,  must  exhibit  maximum  coloration.  Again  Heyn  has 
shown  that  the  solubility  of  steel  in  dilute  sulphuric  acid  increases  with  the  amount 
of  troostite  present,  being  maximum  in  steel  tempered  to  about  400  deg.  C.  From 


1000 


2.00 


Hardened  Sfeel 


Fig.  2.  —  Diagram  depicting  the  constituents  formed  (I)  on  slow  cooling,  (II)  on  quick  cooling,  and 

(III)  on  reheating  hardened  steel. 


these  observations  Heyn  described  osmondite  as  a  constituent  of  steel  characterized 
by  maximum  solubility  in  acids  and  by  maximum  coloration  under  the  action  of  acid 
metallographic  reagents.  In  the  report  of  the  Committee  on  the  Nomenclature  of 
the  Constituents  of  Iron  and  Steel  of  the  International  Association  for  Testing  Mate- 
rials osmondite  is  described  as  follows:  "Probably  aggregate.  That  stage  in  the 
transformation  of  austenite  at  which  the  solubility  in  dilute  sulphuric  acid  reaches  its 
maximum  rapidity.  Arbitrarily  taken  as  the  boundary  between  troostite  ami  sor- 
bite  .  .  .  The  following  hypotheses  have  been  suggested,  none  of  which  has  sub- 
stantial experimental  foundation:  (1)  A  solid  solution  of  carbon  or  an  iron  carbide  in 
alpha  iron;  (2)  The  colloidal  system  of  Benedicks  in  its  purity,  troostite  being  this 
system  while  forming  at  the  expense  of  martensite,  and  sorbite  being  this  system 


LESSON  XIV  --THE   TEMPER  ING   OF   HARDENED   STEEL  7 

coagulating  and  passing  into  pearlite;  (3)  The  stage  of  maximum  purity  of  amorphous 
alpha  iron  in  the  way  to  crystallizing  into  ferrite." 

Structural  Changes  on  Slow  Cooling,  Quick  Cooling,  and  Reheating.  —  It  seems 
helpful  and  instructive  to  depict  graphically  in  a  single  diagram  the  structural  changes 
taking  place  in  eutectoid  steel  (1)  on  slow  cooling  through  its  critical  range,  (2)  on 
quick  cooling  through  that  range,  and  (3)  on  reheating  quickly  cooled  (hardened)  steel 
above  the  range.  The  changes  indicated  in  I  (Fig.  2)  show,  as  already  explained,  that 
steel,  on  cooling  slowly  through  the  critical  range,  is  converted  successively  into  mar- 
tensite,  troostite,  sorbite,  and  pearlite.  On  heating  the  same  steel  from  below  to 
above  the  range  the  same  changes  would  take  place  but  in  the_reyerse  order.  In  II 
the  steel  has  been  cooled  through  the  range  at  such  speed  that  martensite  was  formed 
but  prevented  from  further  transformation,  hardened  martensitic  steel  being  pro- 
duced. The  reheating  of  this  martensitic  steel  is  depicted  in  III.  Below  the  range 
martensite  is  gradually  converted  first  into  troostite  and  then  into  sorbite.  On  enter- 
ing the  range  the  steel  remains  sorbitic  but  on  further  heating  the  sorbite  is  converted 
back  into  troostite  and  then  into  martensite.  Near  the  top  of  the  range  austenite 
begins  forming,  the  transformation  being  complete  as  the  steel  emerges  from  its  range. 
Similar  structural  transformations  would  take  place  in  subjecting  hypo-eutectoid 
and  hyper-eutectoid  steels  to  like  treatments,  but  free  ferrite  or  free  cementite  would 
generally  be  present. 

Microstructure  of  Hardened  and  Tempered  Steel.  —  The  structural  changes 
corresponding  to  the  transformations  taking  place  on  tempering  hardened  steel  de- 
scribed in  the  foregoing  pages  are  not  always  readily  detected  by  microscopical  ex- 
amination. This  is  due  to  the  fact  that,  structurally  speaking,  these  changes  are  often 
pseudomorphic  changes,  the  crystalline  forms  of  the  original  constituent  or  constit- 
uents having  been  retained,  although  the  nature  of  the  crystals  themselves  has  been 
altered. 

Referring  to  pseudomorphism  Dana  writes,  "The  crystalline  forms  under  which  a 
species  occur  are  sometimes  those  of  another  species."  Bayley  defines  pseudomorphs 
as  bodies  possessing  forms  borrowed  from  another  substance,  or  as  a  body  possessing 
the  form  of  one  substance  and  the  chemical  and  physical  properties  of  another.  In 
the  formation  of  a  pseudomorph  the  material  of  the  original  substance  is  replaced 
by  the  new  substance  but  its  external  form  remains  unchanged. 

According  to  Heyn  the  following  appearances  are  observed  in  the  case  of  eutectoid 
steel : 

(1)  After  hardening  but  before  tempering:  martensite  with  well-developed  needles 
remaining  uncolored  after  an  immersion  of  ten  minutes  in  a  solution  of  one  per  cent  of 
hydrochloric  acid  in  alcohol. 

(2)  After  tempering  between  100  and  200  deg. :  martensitic  structure  unchanged 
but  colored  yellow  or  brown. 

(3)  After  tempering  to  275  deg. :  the  needle  structure  becomes  coarser  and  recalls 
mixtures  of  austenite  and  martensite,  one  of  the  constituents  remaining  uncolored, 
the  other  assuming  a  dark  coloration. 

(4)  After  tempering  to  405  deg. :  the  needles  have  disappeared  and  the  sample 
appears  dark  and  mottled  suggesting  a  mixture  of  two  relatively  dark  constituents; 
this  is  troostite. 

(5)  After  tempering  to  500  deg. :  the  light  areas  become  more  abundant. 

(6)  After  tempering  to  GOO  deg. :  irregular  meshes  are  observed  rounded  and  light, 
partially  surrounded  by  a  darker  network  which  appear  to  stand  in  relief. 


8  LESSON   XIV— THE   TEMPERING   OF   HARDENED   STEEL 

Photomicrographs  of  the  structure  of  hardened  and  tempered  steels  are  repro- 
duced in  Figures  3  and  4.  The  structure  of  hardened  steel  reheated  to  600  deg. 
has  been  shown  in  Figure  13,  Lesson  XII. 

Carbon  Condition  in  Tempered  Steel.  —  It  is  generally  admitted  that  the  com- 
bined carbon  present  in  steel  may  exist  under  two  different  conditions,  (1)  as  harden- 
ing carbon,  that  is,  as  carbon  or  the  carbide  Fe3C  dissolved  in  iron,  and  (2)  as  cement 
carbon,  that  is,  as  the  crystallized  carbide  FesC  or  cementite.  It  is  further  believed 
(a)  that  pearlite  is  quite,  if  not  altogether,  free  from  hardening  carbon,  (6)  that  mar- 
tensite  contains,  for  a  given  steel,  the  maximum  amount  of  hardening  carbon  that  can 
be  produced  and  retained  in  that  steel  by  the  ordinary  hardening  operation,  and 


Fig.  3.  —  Steel.  Carbon  0.45  per  cent.  .Minified  1000  diameters. 
Heated  to  825  deg.  C.,  quenched  from  720  deg.,  and  tempered  between 
blue  and  brown  (275  deg.?).  (Osmond.) 

(c)  that  on  tempering  martensite  the  amount  of  hardening  carbon  decreases  as  the 
tempering  temperature  increases,  while  the  proportion  of  cement  carbon  increases 
correspondingly.  Heyn,  however,  found  that  on  analyzing  the  residue  remaining  on 
dissolving,  in  dilute  sulphuric  acid,  hardened  eutectoid  steel  tempered  below  400  deg. 
it  contained  no  carbon  in  the  form  of  the  carbide  Fe3C,  that  is  no  cement  carbon. 
For  the  carbon  remaining  in  the  residue  and  which,  in  his  opinion,  is  different  from 
cement  carbon,  Heyn  suggested  the  notation  Cf.  Not  until  a  temperature  of  400 
had  been  reached  on  tempering  was  cement  carbon  detected  in  the  residue.  Heyn 
infers  from  these  observations  (1)  that  troostite  contains  no  cement  carbon,  its  resid- 
ual carbon  being  in  the  hypothetical  Cf  condition,  (2)  that  osmondite  contains  the 
maximum  amount  of  Cf  carbon,  and  (3)  that  sorbite  must  be  formed,  that  is,  the  steel 
must  be  tempered  above  400  deg.  in  order  to  produce  cement  carbon.  Heyn  further 
argues  that  the  deep  coloration  produced  on  etching  hardened  and  tempered  steel  is 
caused  by  the  separation  of  Cf  carbon,  being,  therefore,  maximum  in  osmondite,  that 
is,  when  the  steel  contains  nothing  but  troostite. 


LESSON   XIV  — THE   TEMPERING   OF   HARDENED   STEEL  9 

Osmond  very  appropriately  remarks  that  it  is  not  necessary  in  order  to  explain 
Heyn's  results  to  admit  the  existence  of  a  new  form  of  carbon,  it  being  quite  possible 
that  the  Cf  carbon  of  Heyn  is  cement  carbon,  that  is,  Fe3C  so  finely  divided  when 
formed  below  400  deg.  that  it  is  readily  decomposed  by  the  acid,  while  above  400  it 
becomes  coarser  and,  therefore,  resists  better  the  action  of  the  acid. 

Decrease  of  Hardness  on  Tempering.  —  According  to  Boynton  the  decrease  of 
hardness  taking  place  on  tempering  is  gradual  up  to  350  deg.,  quite  sudden  between 
350  and  550,  and  nil  above  550  deg. 

Heyn  found  the  following  values  for  the  loss  of  hardness  on  tempering  expressed 
in  per  cent  of  the  original  increase  produced  by  the  hardening  operation: 

at  100  deg.    2.5  per  cent  at  400  deg.  70.0  per  cent 

"  200     "     14.0    "      "  "  500     "    87.5    "      " 

"  300     "    41.0    "      "  "  600     "    97.5    "      " 

Heyn  also  observed  that  the  loss  of  hardness  takes  place  most  quickly  at  300  deg. 


Fig.  4.  —  Steel.  Carbon  0.50  per  cent.  Magnified 
500  diameters.  Heated  to  850  deg.  C.,  quenched  in 
water,  reheated  to  400  deg.,  and  quenched  in  water. 
(\V.  H.  Knight  in  the  author's  laboratory.) 

Heat  Liberated  on  Tempering.  —  In  hardening  steel  the  cooling  through  and 
below  its  critical  range  is  so  rapid  that  the  transformation  of  austenite  which  would 
have  taken  place  had  time  been  given  can  proceed  but  partially,  namely,  to  the  mar- 
tensitic  or  troosto-martensitic  stage.  The  heat  which  would  have  been  generated  had 
the  transformation  been  complete  remains  latent  in  hardened  steel.  On  tempering, 
however,  the  partially  suppressed  transformation  is  permitted  to  proceed  farther, 
this  return  to  a  more  stable  condition  being  accompanied  by  some  evolution  of  heat. 
According  to  Osmond  this  latent  heat  can  be  made  apparent  by  dissolving  hardened 
steel  in  double  chloride  of  ammonium  and  copper  when  it  evolves  more  heat  than 
unhardened  steel. 

Heyn  made  some  careful  determinations  of  the  heat  generated  on  tempering 
hardened  steel.  The  greater  acceleration  in  heating  hardened  steel  was  made  apparent 


10  LESSON   XIV  — THE   TEMPERING   OF   HARDENED   STEEL 

by  the  differential  method,  using  as  neutral  bodies  similar  steels  in  their  pearlitie,  that 
is  unhardened,  condition.  It  was  observed  that  the  heat  generated  on  tempering  is 
maximum  at  360  deg. 

Experiments 

A  bar  of  steel  6  inches  long  and  about  \/l2.  inch  square  or  round,  preferably  of  eutec- 
toid  composition,  should  be  heated  well  above  its  critical  range  and  quenched  in  cold 
water.  It  should  then  be  freed  from  scale  and  brightened  by  rubbing  it  with  emery- 
paper,  and  one  end  of  it  heated  in  the  flame  of  a  bunsen  burner.  At  some  distance 
from  the  heated  end  the  tempering  colors  will  be  observed  in  their  usual  order. 

Pieces  should  now  be  detached  from  the  treated  bar  corresponding  to  several 
tempering  colors,  for  instance,  yellow,  brown,  and  blue,  as  well  as  a  piece  near  the 
cold  end  and  therefore  not  tempered.  These  should  be  polished,  etched,  and  micro- 
scopically examined.  In  polishing  hardened  steel  great  care  must  be  taken  to  prevent 
the  heating  of  the  samples,  since  this  would  necessarily  cause  a  certain  amount  of 
tempering  and  therefore  of  structural  transformation.  A  liberal  supply  of  water 
should  be  used. 

Two  small  pieces  of  the  same  steel  should  be  hardened  and  reheated,  one  to 
400  deg.  C.  and  the  other  to  600  deg.  The  first  sample  should  contain  a  great  deal  of 
troostite,  the  last  should  be  sorbitic. 

All  samples  should  be  photographed. 

Examination 

Describe  the  tempering  (1)  of  martensitic  steel,  (2)  of  troostitic  steel,  and  (3)  of 
troosto-martensitic  steel. 


LESSON  XV 

THEORIES  OF  THE  HARDENING  OF  STEEL 

Many  theories  have  been  put  forward  to  explain  the  hardening_of  steel  through 
sudden  cooling  from  a  high  temperature.  They  may  be  divided  into  two  classes, 
(I)  the  "retention"  theories  and  (II)  the  stress  theory.  The  retention  theories  in- 
clude (A)  the  solution  theories  and  (B)  the  carbon  theories.  Three  solution  theories 
at  least  have  been  proposed,  (a)  the  gamma  iron  theory,  (6)  the  beta  iron  theory,  and 
(c)  the  alpha  iron  theory,  while  two  carbon  theories  should  be  mentioned,  (a)  the  hard- 
ening carbon  theory  and  (b)  the  subcarbide  theory.  This  classification  of  the  harden- 
ing theories  is  given  below  in  a  tabular  form,  as  well  as  the  names  of  their  proposers 
and  of  some  well-known  scientists  supporting  them. 


(a)  Gamma  Iron 

Edwards 

theory 

(6)  Beta  iron  or 

Osmond,  Roberts-Austen, 

(A)  Solution 

allotropic 

Howe,   and  a  majority 

theories 

theory 

of  writers 

(I)  Retention 

(c)   Alpha  iron 

Le  Chatelier,  Guillet 

theories 

theory 

(a)  Hardening 

(B)  Carbon 

carbon  theory 

theories 

(6)  Subcarbide 

Arnold 

theory 

(II)  Stress 

theory 


I  Andre  Le  Chatelier 
Charpy,  Grenet 


Retention  Theories.  —  The  retention  theories  claim  that  in  hardening  steel  a  con- 
dition or  set  of  conditions  existing  normally  above  its  critical  range  is  retained  un- 
changed in  the  cold  or  but  partially  changed  because  of  the  rapid  cooling  through  and 
below  the  critical  range.  In  other  words,  such  very  quick  cooling  through  the  range 
denies  the  necessary  time  for  the  transformations  to  take  place,  at  least  fully,  and  a 
condition  is  preserved  in  the  cold  which  is  stable  only  above  or  within  the  critical 
range.  According  to  these  theories  hardened  steel,  therefore,  is  in  an  unstable  condi- 
tion, hence  the  possibility  of  tempering  it.  That  the  transformations  which  would 
have  taken  place  on  slow  cooling  through  the  range  are  suppressed,  partly  at  least,  in 
hardening  is  made  evident  by  the  absence  of  critical  points  during  very  rapid  cooling 
or  by  the  appearance  of  feeble  points  only  at  greatly  lowered  temperatures.  Other 
evidences  of  the  partial  suppression  of  the  transformations  are  afforded  (1)  by  the 

1 


2  LESSON  XV  — THEORIES  OF  THE  HARDENING  OF  STEEL 

condition  of  the  carbon  in  hardened  steel  which  is  different  from  the  condition  of 
that  element  in  unhardened  steel,  (2)  by  the  evolution  of  heat  taking  place  on  tem- 
pering hardened  steel  as  explained  in  Lesson  XIV,  and  (3)  by  the  structure  of 
hardened  steel. 

To  assume  that  hardened  steel  owes  its  hardness  to  the  suppression,  partial  at 
least,  of  the  transformations  taking  place  on  slow  cooling  through  the  critical  range 
is,  therefore,  both  natural  and  logical. 

When  we  come  to  look  into  the  nature  of  the  constituents  stable  above  or  within 
the  range,  which  on  being  retained  in  the  cold  impart  extreme  hardness  to  steel,  we 
find  very  great  differences  of  opinion  among  competent  authorities. 

Solution  Theories.  —  The  majority  of  writers  believe  that  hardened  steel  is  in  the 
condition  of  a  solid  solution,  quick  cooling  through  the  range  having  prevented  the 
formation  of  the  ferrite-cementite  aggregate.  This  is  strongly  supported  by  micro- 
scopical and  other  evidences.  They  all  agree  that  the  solution  consists  of  carbon  dis- 
solved in  iron  but  different  views  are  held  in  regard  to  the  condition  of  the  carbon  and 
of  the  iron.  While  it  is  now  the  general  belief  that  the  carbide  FesC  rather  than  ele- 
mentary carbon  is  dissolved  in  iron,  some  think  that  in  hardened  steel  the  iron  is 
chiefly  present  as  gamma  iron,  others  that  it  is  present  chiefly  as  beta  iron,  while  others 
still  believe  that  it  exists  mainly  in  the  alpha  condition.  These  three  contentions  will 
be  briefly  considered. 

Gamma  Iron  Theory.  —  Edwards  claims  that  on  rapid  cooling  through  the  range 
iron  is  retained  in  the  gamma  condition.  In  other  words  that  martensite,  the  usual 
constituent  of  hardened  steel,  is  identical  in  composition  if  not  in  structure  to  austen- 
ite,  the  solid  solution  of  carbon  in  gamma  iron  stable  above  the  range.  His  conten- 
tion is  based  solely  upon  the  absence  of  the  point  A2  in  medium  high  and  in  high 
carbon  steel  from  which  he  argues  that  beta  iron  does  not  form  in  these  steels,  dis- 
missing the  very  reasonable  possibility  of  beta  iron  forming  at  the  points  As.2  and 
Aa.g.i.  Edwards'  theory  does  not  explain,  at  least  satisfactorily,  the  marked  difference 
of  hardness  between  austenite  and  martensite  nor  their  totally  different  structure. 
Benedicks,  moreover,  has  shown  quite  conclusively  that  austenite  cannot  exist  in 
the  cold  unless  subjected  to  very  great  pressure,  as  explained  in  Lesson  XIII. 
Razor  blades,  for  instance,  although  made  extremely  hard  by  quenching  cannot 
be  austenitic. 

Beta  Iron  or  Allotropic  Theory.  —  This  theory  put  forward  with  great  vigor  and 
brilliancy  by  Osmond,  championed  first  by  the  late  Roberts-Austen  and  later  by 
Howe  and  many  other  eminent  metallurgists,  contends  that  in  hardened  steel  iron  is 
present  chiefly  in  the  beta  condition.  It  is  often  referred  to  as  the  allotropic  theory 
of  the  hardening  of  steel.  It  should  be  borne  in  mind,  however,  that  Osmond's  allo- 
tropic theory  of  iron  and  his  allotropic  theory  of  the  hardening  of  steel  are  two  differ- 
ent conceptions.  One  may  believe  in  the  former  without  being  an  adherent  of  the 
latter.  The  allotropic  theory  of  iron  claims  that  iron  exists  in  at  least  two  and  prob- 
ably in  three  allotropic  forms.  No  one  doubts  the  allotropy  of  iron  although  some 
writers,  notably  Le  Chatelier,  believe  that  only  two  allotropic  forms,  namely  gamma 
and  alpha  iron,  have  been  shown  to  exist.  The  allotropic  theory  of  the  hardening  of 
steel  claims  (1)  that  in  hardened  steel  carbon,  or  more  probably  the  carbide  FeaC,  is 
in  solution  chiefly  in  beta  iron,  hence  its  hardness,  beta  iron  being  very  hard,  (2)  Iliat 
hardened  steel  contains  alpha  iron,  hence  its  magnetism,  alpha  iron  being  the  only 
allotropic  form  under  which  iron  is  magnetic. 


LESSON  XV  — THEORIES  OF  THE  HARDENING  OF  STEEL  3 

While  the  allotropists  regard  the  retention  of  beta  iron  as  the  chief  cause  of  the 
hardening  of  steel,  they  do  not  ignore  the  very  important  part  played  by  carbon. 
They  realize  that  the  presence  of  carbon  is  essential  to  the  retention  of  iron  in  its  hard 
allotropic  form,  for  in  the  absence  of  carbon  it  is  not  possible  to  harden  iron.  It  is 
customary  to  compare  this  action  of  carbon  in  preventing  the  transformations  to  that 
of  a  brake,  the  more  carbon  the  more  powerful  the  brake  action,  hence  the  harder  the 
steel  because  of  the  retention  of  a  larger  quantity  of  beta  iron.  They  believe,  however, 
that  the  hardness  of  quenched  steel  increases  with  the  proportion  of  the  beta  iron  it 
contains  rather  than  with  the  proportion  of  carbon.  In  other  words,  that  if  it  were 
possible  to  retain  the  same  amount  of  beta  iron  with  less  carbon  or  even  in  the  com- 
plete absence  of  carbon  the  metal  would  be  equally  hard.  The  ajlotropists  do  not 
claim  that  steels  hardened  in  the  ordinary  way,  that  is,  martensitic  steels,  are  abso- 
lutely free  from  gamma  iron.  Evidences  are  lacking  to  settle  this  point.  Again  the 
allotropists  do  not  deny  that  the  internal  pressure  created  by  the  transformation  of 
austenite  into  the  more  bulky  martensite  may  contribute  to  the  hardness  of  the 
metal. 

Summing  up,  in  the  light  of  this  theory,  the  hardening  of  steel  by  rapid  cooling  is 
thus  explained:  (1)  the  bulk  of  the  iron  passes  from  the  gamma  to  the  beta  condi- 
tion, hence  the  great  hardness  produced,  (2)  some  of  the  beta  iron  is  further  trans- 
formed into  alpha  iron,  hence  the  magnetism  of  hardened  steel,  (3)  a  large  proportion 
of  the  carbon  or  more  probably  of  the  carbide  FesC  remains  dissolved  in  the  beta 
iron,  the  presence  of  this  dissolved  (hardening)  carbon  in  hardened  steel  being  proven 
by  chemical  analysis,  (4)  the  internal  pressure  created  by  the  transformation  of  aus- 
tenite into  martensite,  that  is,  of  gamma  into  beta  iron,  may  contribute  to  the  final 
hardness.  Osmond's  theory  of  the  hardening  is,  therefore,  based  on  the  belief  (1)  in 
the  existence  of  beta  iron  and  (2)  in  the  hardness  of  beta  iron.  Although  the  very 
existence  of  beta  iron  has  been  challenged  by  Le  Chatelier  and  Guillet  the  author 
believes  that  the  evidences  at  hand  point  strongly  to  its  reality,  while  direct  evidences 
of  its  hardness  have  been  obtained.  Rosenhain  and  Humfrey  on  straining  polished 
bands  of  pure  iron  heated  in  vacuum  obtained  three  sharply  distinct  structures,  each 
one  possessing  different  mechanical  properties,  especially  as  to  hardness,  and  each 
structure  representing  the  condition  of  the  iron,  respectively,  above  A3,  between  A3 
and  A2,  and  below  A2,  that  is,  the  structure  of  gamma,  beta,  and  alpha  iron.  The 
iron  when  in  the  beta  condition  was  found  to  be  decidedly  harder  than  either  gamma 
or  alpha  iron.  These  experiments  point,  therefore,  both  to  the  existence  of  beta  iron 
and  to  its  hardness. 

On  tempering  hardened  steel  Heyn  found  that  70  per  cent  of  the  increased  hard- 
ness produced  by  the  hardening  operation  were  lost  in  tempering  below  400  deg., 
the  remaining  30  per  cent  being  possibly  due,  according  to  Osmond,  to  the  internal 
pressure  already  alluded  to.  He  further  observed  that  hardening  (dissolved)  carbon 
did  not  begin  to  be  converted  into  cement  carbon  (crystallized  Fe3C)  until  a  tempera- 
ture of  400  deg.  was  reached.  Steel  then  loses  most  of  its  hardness  while  its  carbon 
remains  in  the  hardening  condition.  From  this  coexistence  of  softness  and  harden- 
ing carbon  it  logically  follows  that  steel  does  not  owe  its  hardness  to  the  presence  of 
hardening  carbon,  and  that  the  presence  of  allotropic  beta  iron  remains  the  only 
possible  explanation.  This  conclusion  is  further  supported  by  the  fact  that  on  tem- 
pering steel  it  is  chiefly  below  400  deg.  that  heat  is  liberated  and  this  liberation  must 
necessarily  be  ascribed  to  the  iron  returning  from  the  beta  to  the  alpha  condition. 


4  LESSON  XV  — THEORIES  OF  THE  HARDENING  OF  STEEL 

Alpha  Iron  Theory.  —  Le  Chatelier  and  Guillet  believe  that  on  quick  cooling 
through  the  critical  range  the  allotropic  transformation  of  iron  from  its  gamma  to 
its  alpha  condition  is  not  prevented  but  that  the  steel  remains,  nevertheless,  in  the 
condition  of  a  solid  solution,  hardened  steel  in  their  opinion  being  a  solid  solution  of 
carbon  (or  of  the  carbide  Fe3C)  in  alpha  iron  owing  its  hardness  to  its  state  of  solu- 
tion and  its  magnetism  to  the  presence  of  alpha  iron.  This  view  is  based  chiefly 
upon  these  writers'  belief  that  the  point  A2  is  not  an  allotropic  point  and  that  there- 
fore beta  iron  does  not  exist.  The  most  serious  objection  to  this  theory  lies  in  the 
conclusive  nature  of  the  evidences  pointing  to  the  allotropic  character  of  the  point  A2. 

Carbon  Theories.  —  The  carbon  theories  contend  that  the  hardness  of  rapidly 
cooled  steel  is  due  primarily  to  the  retention  in  the  cold  of  a  very  hard  condition  of 
the  carbon  normally  stable  only  above  the  range,  the  allotropic  transformation  of 
iron  playing  no,  or  but  an  unimportant,  part  in  the  phenomenon.  As  supporting 
their  claims  they  point  to  the  fact  that  carbonless  iron  cannot  be  hardened  and  that 
the  more  carbon  present  the  greater  the  increased  hardness  produced  by  quick  cool- 
ing, at  least  up  to  the  eutectoid  carbon  ratio. 

These  theories  differ  in  regard  to  the  exact  condition  of  the  carbon  thus  retained 
by  quenching  and  imparting  great  hardness  to  the  metal.  The  hardening  carbon 
theory  and  the  subcarbide  theory  should  be  briefly  described. 

The  Hardening  Carbon  Theory.  —  It  was  held  for  many  years  by  the  majority  of 
writers  that  hardened  steel  owed  its  hardness  to  the  presence  of  hardening  carbon,  a 
form  of  carbon  stable  only  above  the  range  but  which  could  be  retained,  in  part  at 
least,  by  quick  cooling.  This  belief  rested  on  the  apparent  difference  existing  between 
the  condition  of  the  carbon  in  hardened  and  in  unhardened  steels  as  proven  by  dis- 
solving them  in  cold  dilute  acids  when  a  large  proportion  of  the  carbon  of  hardened 
steel  escapes  as  hydrocarbons,  whereas  nearly  the  totality  of  the  carbon  of  unhard- 
ened steel  remains  as  a  residue  which,  upon  being  analyzed,  is  found  to  consist  of  the 
carbide  FesC.  As  to  the  exact  nature  of  hardening  carbon,  vague,  conflicting,  and 
often  extraordinary  statements  appeared,  it  being  claimed  by  some,  for  instance,  that 
hardening  carbon  was  carbon  in  a  diamond-like  condition.  It  is  at  present  believed 
by  most  that  hardening  carbon  is  carbon  (or  more  probably  the  carbide  Fe3C)  dis- 
solved in  iron,  its  escape  as  hydrocarbons  upon  being  subjected  to  the  action  of  dilute 
acids  being  due  to  its  extremely  fine  state  of  division.  If  this  be  the  nature  of  harden- 
ing carbon,  the  hardening  carbon  theory  becomes,  of  course,  a  solution  theory. 

It  is  obvious  that  carbon  as  such,  no  matter  how  great  its  hardness,  could  not 
impart  extreme  hardness  to  steel  in  which  it  may  be  associated  with  199  times  its 
weight  of  soft  ferrite,  as,  for  instance,  in  steel  containing  0.50  per  cent  carbon.  The 
contention  that  it  is  not  carbon,  as  such,  which  is  retained  by  quick  cooling  but  a  very 
hard  carbide  constituting  the  whole  or  a  large  part  of  hardened  steel  is  not,  of  course, 
open  to  the  same  objections.  This  is  the  claim  of  the  subcarbide  theory. 

The  Subcarbide  Theory.  —  Arnold  contends  that  eutectoid  steel  above  its  critical 
range  exists  as  the  carbide  Fe^C,  a  chemical  compound  containing  about  0.89  per 
cent  carbon.  This  carbide  which  he  calls  "hardenite"  is  very  hard  and  being  re- 
tained by  quick  cooling  imparts  hardness  to  quenched  steel.  In  hypo-eutectoid  steel 
some  ferrite  and  in  hyper-eutectoid  steel  some  cementite  are  dissolved  in  this  sub- 
carbide.  It  follows  from  this  theory  that  austenite  and  martensite  correspond  to 
different  structural  appearances  of  the  same  constituent,  namely,  the  carbide  Fe2.jC 
when  of  eutectoid  composition,  the  same  carbide  plus  ferrite  or  cementite  in  hypo-  or 


LESSON  XV  — THEORIES  OF  THE  HARDENING  OF  STEEL  5 

hyper-eutectoid  steel.  This  theory  is  purely  of  a  speculative  character,  the  existence 
of  the  carbide  Fe24C  not  being  supported  by  a  single  direct  evidence.  It  is,  moreover, 
strongly  opposed  by  the  universally  accepted  theory  of  metallic  alloys  which  holds 
that  eutectic  (and  eutectoid)  alloys  immediately  before  their  formation  are  not  defi- 
nite chemical  compounds  but  liquid  or  solid  solutions.  On  forming,  whether  or 
not  it  implies  a  change  of  state,  the  solution  is  transformed  into  an  aggregate  of  the 
solute  and  solvent,  ferrite  and  cementite  in  the  case  of  iron-carbon  alloys.  The 
breaking  up  at  a  certain  critical  temperature  of  a  definite  chemical  compound, 
as  demanded  by  Arnold's  theory,  into  a  eutectoid  aggregate  of  the  elements  of  that 
compound  is  contrary  to  our  firmly  established  knowledge  of  the  mechanism  of  the 
formation  of  such  aggregates.  It  implies  a  return  to  Guthrie's  original  error. 

The  Stress  Theory.  —  In  cooling  steel  quickly  from  above  its  critical  range  it  is 
subjected  to  two  kinds  of  stresses,  (1)  stresses  due  to  the  shrinkage  of  its  outer  shell 
on  its  interior  and  (2)  stresses  due  to  the  transformation  with  increased  volume  of 
gamma  into  beta  and  alpha  iron.  The  existence  of  the  strains  resulting  from  these 
stresses  have  been  claimed  to  account  satisfactorily  for  the  hardening  of  steel  by 
sudden  cooling.  It  was  argued  long  ago,  for  instance,  that  the  hardening  of  steel  by 
sudden  cooling  might  be  due  to  the  metal  being  in  a  severely  strained  condition,  be- 
cause of  the  quicker  cooling  of  the  outer  layers,  these  layers  through  their  contrac- 
tion exerting  a  severe  pressure  upon  the  central  portion  of  the  steel  objects.  The 
advocates  of  this  theory  pointed  to  the  increased  hardness  resulting  from  cold  work- 
ing steel  as  a  proof  that  severe  straining  produces  hardness.  Some  went  as  far  as  to 
claim  that  cold  worked  steel  and  hardened  steel  are  practically  in  the  same  physical 
condition,  the  quenching  of  steel  producing,  so  to  speak,  an  internal  cold  working 
(straining)  of  the  metal.  They  seem  to  have  overlooked  the  enormous  difference  be- 
tween the  relatively  small  increase  of  hardness  produced  by  cold-working  and  the 
hardness  resulting  from  quenching.  In  the  fact  that  both  cold-working  and  harden- 
ing increase  the  elastic  limit  and  decrease  the  ductility  they  found  additional  support 
for  their  view.  According  to  Osmond  the  belief  once  held  that  cold  working  causes 
an  allotropic  transformation  is  now  abandoned.  While  it  is  not  unreasonable  to 
assume  that  the  strained  condition  of  the  metal  adds  to  its  hardness  it  is  hardly  think- 
able that  the  sudden  and  very  great  increase  of  hardness  produced  by  quick  cooling 
is  due  altogether  to  this  straining.  If  it  were  so  the  outer  layers  of  quenched  steel 
implements  should  not  be  hard  and  razor  blades  could  not  be  hardened.  Again,  it  is 
impossible  to  reconcile  this  theory  with  the  fact  that  it  is  necessary  to  quench  steel 
from  above  its  critical  range  in  order  to  harden  it,  for  one  cannot  conceive  why,  if  the 
steel  be  quenched  slightly  below  the  range,  the  strain  created  in  the  quenching  bath 
would  be  so  slight  as  to  have  no  hardening  effect,  whereas  quenching  from  a  tempera- 
ture but  a  few  degrees  higher  would  produce  very  severe  straining.  Finally,  if  the 
contraction  of  the  outer  layers  due  to  their  rapid  cooling  can  induce  such  hardening 
strains  in  the  case  of  steel,  it  is  surprising  that  a  similar  phenomenon  is  not  observed 
in  the  case  of  other  metals.  The  internal  strains  resulting  from  the  transformation 
of  gamma  into  beta  iron  (austenite  into  martensite)  with  increased  volume  afford  a 
more  acceptable  explanation  of  the  hardening  of  steel.  It  was  offered  long  ago  and 
has  recently  been  revived  and  presented  in  a  more  scientific  way,  notably  by  Andre 
Le  Chatelier,  Charpy,  and  Grenet.  It  is  argued  that  on  quick  cooling  the  allotropic 
transformations  take  place,  partially  at  least,  at  a  temperature  so  low  that  the  metal 
lacks  the  necessary  plasticity  to  yield  to  the  severe  stress  excited  by  these  transforma- 


6  LESSON  XV  — THEORIES  OF  THE  HARDENING  OF  STEEL 

tions,  remaining,  therefore,  severely  strained.  It  becomes  internally  "ecroui,"  as  the 
French  express  it.  Here  again,  however,  it  would  seem  as  if  the  outer  layers  should 
not  be  strained  and  should,  therefore,  remain  soft,  which  of  course  is  contrary  to  facts. 
Nor  is  the  failure  of  carbonless  iron  to  harden  satisfactorily  explained  by  this  theory. 

Grenet  believes  that  in  hardened  steel  the  allotropic  transformations  are  complete, 
that  is,  that  its  iron  exists  only  in  the  alpha  condition  but  so  severely  strained  (ecroui) 
as  to  be  very  hard.  He  rests  his  belief  chiefly  on  his  assertion  that  on  quick  cooling 
the  dilatation  indicative  of  the  allotropic  transformations  are  not  suppressed  and 
that  the  metal  does  not  remain  non-magnetic.  He  overlooks  the  claims  of  the  advo- 
cates of  the  retention  theories,  so  strongly  supported,  that  the  transformations  are 
not  completely  suppressed,  hence  the  occurrence  of  a  dilatation  and  of  magnetism. 
When  they  are  completely  prevented,  as  in  austenitic  steels,  the  metal  neither  expands 
nor  becomes  magnetic  on  cooling. 

The  liberation  of  heat  observed  on  tempering  hardened  stee.1  points  to  a  return  to 
a  more  stable  condition,  supporting,  therefore,  the  retention  theories  and  opposing  the 
stress  theory. 

Tempering  and  the  Retention  Theories.  —  The  tempering  of  hardened  steel,  as 
already  explained,  is  readily  accounted  for  by  the  retention  theories  on  the  ground 
that  the  metal  being  in  an  unstable  condition  is  ever  eager  to  assume  a  more  stable 
form,  implying  a  return,  partial  at  least,  of  the  iron  to  the  alpha  condition  and  of  the 
carbon  to  the  cement  condition.  On  heating  the  steel  but  slightly  above  atmospheric 
temperature  its  rigidity  is  sufficiently  diminished  to  permit  a  slight  transformation 
of  this  kind,  the  higher  the  temperature  the  more  pronounced  of  course  being  its 
tempering. 

Tempering  and  the  Stress  Theory.  —  The  stress  theory,  likewise,  satisfactorily 
accounts  for  the  tempering  of  hardened  steel  on  the  ground  that  upon  slight  reheat- 
ing the  internal  strains  are  sufficiently  released  to  produce  an  appreciable  decrease  of 
the  specific  effects  of  hardening,  namely,  decrease  of  hardness,  of  strength,  of  elastic 
limit  and  increased  ductility. 

Summary.  —  From  the  above  short  description  of  the  various  theories  advanced 
to  explain  the  hardening  of  steel  the  reader  will  probably  gather  the  impression  that 
the  retention  theories,  especially  the  beta  iron  theory,  are  the  most  acceptable  ones. 
It  seems  quite  possible,  however,  even  probable,  that  the  various  theories,  while  ap- 
parently antagonistic,  bring  each  their  contribution  to  the  elucidation  of  the  problem. 
Should  we  not  believe  with  the  allotropists  that  the  hardness  of  steel  is  due  chiefly  to 
the  retention  of  a  large  quantity  of  a  hard  allotropic  variety  of  iron,  probably  beta 
iron,  and  that  this  iron  contains  in  solution  the  hardening  carbon  of  the  carbonists, 
the  presence  of  which  is  absolutely  essential  to  the  existence  of  beta  iron  in  the  cold? 
Should  we  not  with  the  advocates  of  the  stress  theories  believe  in  the  hardening  in- 
fluence of  the  strains  created  on  quick  cooling  (a)  because  of  the  shrinkage  of  the 
outer  layers  of  the  metal  and  (6)  because  of  the  expansion  accompanying  the  trans- 
formation of  gamma  into  beta  iron?  None  of  these  theories  alone  gives  a  fully  satis- 
factory explanation:  Beta  iron  cannot  be  retained  in  the  absence  of  carbon  and  if  it 
could  be  it  is  not  certain  that  it  would  be  intensely  hard;  the  presence  of  intensely 
hard  carbon  or  iron  carbide  as  the  chief  cause  of  hardening  is  contrary  to  evidences; 
the  strained  condition  of  hardened  steel  does  not  account  satisfactorily  for  its  hard- 
ness; Le  Chatelier's  contention  that  quickly  cooled  steel  is  hard  although  its  iron  is 
in  the  soft  alpha  condition  because  of  its  being  in  a  state  of  solution  is  opposed  by 


LESSON  XV  — THEORIES  OF  THE  HARDENING  OF  STEEL  7 

the  evidences  at  hand  (a)  of  the  existence  of  beta  iron  and  (6)  of  the  hardness  of  beta 
iron;  Arnold's  theory  that  hardened  steel  owes  its  hardness  to  the  retention  of  a  hard 
subcarbide  of  iron  lacks  experimental  support  and  is  scientifically  untenable. 


Examination 

Describe  briefly  the  various  theories  of  the  hardening  of  steel  indicating  your 
preference  and  your  arguments  supporting  it. 


LESSON  XVI 

THE  CEMENTATION  AND  CASE  HARDENING  OF  STEEL 

The  affinity  of  iron  for  carbon  is  so  great  that  when  heated -to -a  sufficiently  high 
temperature  in  contact  with  some  suitable  carbonaceous  matter  it  readily  absorbs 
carbon.  If  the  heating  be  protracted  (several  days)  and  the  amount  of  carbon  ab- 
sorbed considerable,  the  operation  is  known  as  "cementation"  and  the  resulting  metal 
as  "cemented,"  "converted,"  or  "blister"  steel,  or  in  Sheffield,  England,  as  "blister 
bar,"  while  if  the  treatment  be  of  relatively  short  duration  (a  few  hours)  and  the 
absorption  of  carbon  in  consequence  superficial,  it  is  called  "case  hardening." 

Cementation  is  generally  applied  to  wrought-iron  bars  which  are  afterwards 
melted  (crucible  process)  and  shaped  into  finished  articles  by  casting  or  forging, 
while  case  hardening  is  applied  directly  to  finished  objects  generally  of  low  carbon 
steel.  The  purpose  of  cementation  is  to  introduce  carbon  into  wrought  iron,  thereby 
converting  it  into  steel,  the  subsequent  treatments  (melting,  forging)  producing  a 
uniform  distribution  of  the  carbon,  whereas  the  purpose  of  case  hardening  is  to 
manufacture  steel  objects  with  hard  skins  or  cases  while  retaining  their  soft  and 
tough  centers  or  cores. 

The  quantity  of  carbon  thus  absorbed  by  iron  at  a  high  temperature  but  below  its 
melting-point  depends  chiefly  upon  (1)  the  composition  of  the  iron  or  steel  subjected 
to  carburizing,  (2)  the  carburizing  temperature,  (3)  the  length  of  time  at  that  tem- 
perature, and  (4)  the  nature  of  the  carburizing  material. 

Composition  of  the  Iron  or  Steel  Subjected  to  Carburizing.  —  It  is  probably  true 
that  the  smaller  the  proportion  of  carbon  in  the  iron  the  more  eagerly  will  it  take  up 
carbon,  from  which  it  follows  that  as  the  carburizing  proceeds,  that  is,  as  the  metal 
becomes  more  highly  carburized,  additional  introduction  of  carbon  requires  progres- 
sively longer  time,  the  metal  acting  in  this  way  not  unlike  a  solution  approaching 
its  saturation  point. 

In  the  cementation  process  bars  of  very  pure  wrought  iron  and  in  "case  harden- 
ing" steel  objects  containing  at  the  most  0.20  per  cent  carbon  are  subjected  to  the 
carburizing  treatment.  The  steel  should  not  contain  over  0.25  per  cent  of  manganese 
lest  the  case  be  too  brittle.  The  presence  of  certain  elements  appear  to  hinder  the 
carburizing  operation  while  others  facilitate  it. 

According  to  Guillet  the  absorption  of  carbon  is  favored  by  those  special  elements 
which  exist  as  double  carbides  such  as  manganese,  tungsten,  chromium,  molybdenum, 
and  opposed  by  those  which  form  solid  solutions  with  iron  such  as  nickel,  silicon,  and 
aluminum. 

Carburizing  Temperature.  —  While  it  has  been  claimed  that  iron  below  its  critical 
range  will  absorb  some  carbon  this  absorption,  if  taking  place  at  all,  is  very  slow, 
from  which  it  is  logical  to  infer  that  alpha  iron  has  very  little,  if  any,  dissolving  power 
for  carbon.  In  order  to  produce  quick  and  intense  carburization  the  iron  should  be 
in  its  beta  or,  more  probably,  in  its  gamma  condition,  and  steel,  therefore,  in  the  condi- 

1 


2         LESSON  XVI  —  THE  CEMENTATION  AND  CASE  HARDENING  OF  STEEL 

tion  of  a  solid  solution.  Cementing  and  case  hardening  operations  must  consequently 
be  conducted  above  the  critical  range  of  the  iron  or  low  carbon  steel  treated,  that  is, 
at  a  temperature  exceeding  800  deg.  C.  It  is  also  certain  that  the  higher  the  tem- 
perature the  quicker  will  carbon  be  absorbed  and  the  deeper  will  it  penetrate  into 
the  steel,  that  is,  the  deeper  the  "case."  At  Sheffield,  England,  where  the  cementation 
process  is  used  more  extensively  than  anywhere  else  the  carburizing  temperature  is 
in  the  vicinity  of  950  to  1000  deg.  Most  case  hardening  treatments  are  probably  con- 
ducted in  the  vicinity  of  900  to  950  deg.  C. 

Time  at  Carburizing  Temperature.  —  The  amount  of  carbon  absorbed,  and  there- 
fore the  thickness  of  the  case  as  well,  increases,  of  course,  with  the  length  of  the 
operation  but,  as  already  mentioned,  carburization  takes  place  more  and  more  slowly 
as  the  carbon  content  increases.  The  maximum  amount  of  carbon  which  iron  can 
take  up  while  in  the  solid  state  is  probably  not  far  from  2.50  per  cent,  this,  however, 


Fig.  1.  —  Steel.    Case  hardened.    Magnified  20  diameters. 

requiring  a  protracted  treatment  at  a  very  high  temperature.  While  in  the  manu- 
facture of  blister  steel  considerably  more  than  one  per  cent  of  carbon  is  frequently  in- 
troduced into  the  wrought-iron  bars,  in  carburizing  finished  steel  articles  it  is  seldom 
desired  to  produce  a  case  containing  more  than  one  per  cent  of  carbon  near  the  outside, 
a  superficial,  carburized  layer  of  eutectoid  composition  (0.85  per  cent  C.)  being  gen- 
erally considered  to  yield  the  best  results.  The  length  of  time  needed  to  produce  the 
desired  degree  of  carburization  and  desired  depth  of  case  must  necessarily  depend  upon 
the  nature  of  the  metal,  the  kind  of  carburizing  material  used,  and  the  temperature. 

Distribution  of  the  Carbon.  —  It  will  be  apparent  from  the  nature  of  the  opera- 
tion that  in  this  carburizing  of  solid  iron  carbon  travels  slowly  from  the  outside 
towards  the  center  and  that,  therefore,  the  proportion  of  carbon  absorbed  must 
decrease  from  outside  to  center,  unless  indeed  the  objects  treated  are  very  thin  or 
the  treatment  so  long  and  conducted  at  so  high  a  temperature  as  to  cause  even  the 
center  to  absorb  the  maximum  amount  of  carbon.  The  decrease  of  carbon  as  one 
approaches  the  core  of  the  object  is  well  illustrated  in  Figures  1  and  2.  A  band  of 
hyper-eutectoid  steel  characterized  by  the  presence  of  free  cementite  is  frequently 


LESSON  XVI  — THE  CEMENTATION  AND  CASE  HARDENING  OF  STEEL         3 

noted  (Fig.  1)  followed  by  a  band  of  cutectoid  composition  characterized  by  the 
absence  of  both  free  cementite  and  free  ferrite  and  this  in  turn  is  followed  by  a 
band  showing  abrupt  and  rapid  decrease  of  carbon  characterized  by  an  increasing 
amount  of  free  ferrite. 

In  case  hardening  operations  the  penetration  of  the  carbon  may  be  very  slight 
indeed,  not  exceeding  0.5  mm.,  while  it  may  measure  as  much  as  5  mm.  In  the 
majority  of  instances  the  penetration  does  not  exceed  2  mm.  This  depth  of  pene- 
tration or  thickness  of  case  must  be  regulated  according  to  requirements.  It  will 
depend  upon  temperature,  time,  composition  of  steel,  and  kind  of  carburizing  material. 
Lake  mentions  0.87  mm.  per  hour  as  an  average  speed  of  penetration.  As  already 
stated,  it  is  not  generally  advisable  to  produce  a  case  containing  "more  than  some 
0.90  per  cent  carbon. 


Fig.  2.  —  Steel.     Case  hardened.     Magnified  100  diameters.     (G.  A.  Rein- 
hardt  in  the  author's  laboratory.) 

The  production  of  a  deep  case,  while  at  the  same  time  keeping  the  carbon  content 
of  the  outside  of  the  case  below  one  per  cent,  may  be  brought  about  by  a  rather  long 
treatment  at  a  relatively  low  temperature,  namely,  some  850  deg.  Some  results  ob- 
tained by  Guillet  in  regard  to  the  influence  of  temperature  and  of  time  on  the  depth 
of  penetration  are  shown  graphically  in  Figure  3  as  plotted  by  Bauer.  The  carbu- 
rizing material  used  was  not  stated.  The  full  line  represents  relative  penetrations  at 
1000  deg.  after  different  lengths  of  time,  namely,  one,  two,  four,  and  six  hours,  while 
the  broken  line  represents  the  depths  of  penetration  resulting  from  heating  for  eight 
hours  at  different  temperatures. 

It  will  be  obvious  that  the  process  of  case  hardening  can  be  controlled  by  the 
microscopical  examination  of  test  pieces  much  more  readily  and  accurately  than  by 
chemical  analysis. 

Carburizing  Materials.  —  A  great  variety  of  carbonaceous  materials  are  used  for 
introducing  carbon  in  iron  and  steel  in  the  solid  state.  These  substances  may  be 
solid,  liquid,  or  gaseous.  Solid  materials  are  used  more  extensively  than  liquid  or 


LESSON  XVI  — THE  CEMENTATION  AND  CASE  HARDENING  OF  STEEL 


gaseous  ones,  the  most  important  being  charcoal  (both  wood  and  bone),  charred 
leather,  crushed  bone,  horn,  mixtures  of  barium  carbonate  (40  per  cent)  and  charcoal 
(60  per  cent)  or  of  salt  (10  per  cent)  and  charcoal  (90  per  cent),  both  recommended 
by  Guillet,  and  for  quick  but  very  superficial  hardening,  powdered  potassium  cyanide 
and  potassium  ferro-cyanide  or  mixtures  of  potassium  ferro-cyanide  and  potassium 
bichromate.  A  molten  bath  of  potassium  cyanide  heated  to  850  deg.  and  in  which 


IIOO'C  a 


1000*    6 


900     * 


800      2 


IO  15  2O  25  30  35 

Penetration,  in,  m/m. 

Fig.  3.  —  Temperature  and  time-penetration  curve.     (From  Brearley's 
"The  Heat  Treatment  of  Tool  Steel.") 


•v  s  a  10 

Time,  of  Heating,  (hows) 

Fig.  4.  —  Time-penetration  curve.    (From  Brearley's  "The 
Heat  Treatment  of  Tool  Steel.") 

the  steel  articles  are  immersed  produces  quickly  superficial  but  hard  and  even  cases. 
The  poisonous  character  of  the  escaping  gases,  however,  is  a  serious  objection  to  the 
use  of  this  method.  The  carburizing  of  iron  may  also  be  performed  at  the  proper 
temperature  by  means  of  gases  such  as  illuminating  or  other  coal  or  oil  gases  rich  in 
hydrocarbons.  At  the  Krupp  works  in  Germany  gases  are  used  for  carburizing  the 
faces  of  armor  plates. 

The  relative  merits  of  wood  charcoal,  charred  leather,  and  a  mixture  of  barium 
carbonate  and  of  wood  charcoal  for  carburizing  are  shown  graphically  in  Figure  4,  in 


LESSON  XVI  — THE  CEMENTATION  AND  CASE  HARDENING  OF  STEEL         5 

which  arc  plotted  some  results  obtained  by  Shaw-Scott.  While  wood  charcoal  causes 
a  slow  carburization  it  is  the  best  material  and  the  one  invariably  employed  for  the 
production  of  very  deep  cases  as,  for  instance,  in  making  blister  steel. 

Many  so-called  secret  mixtures  are  offered  for  sale  as  case  hardening  substances 
for  which  extraordinary  virtues  are  claimed,  the  usual  statement  being  that  by  their 
use  steel  of  ordinary  or  inferior  quality  may  be  converted  into  high  grade  metal  com- 
parable to  the  best  crucible  tool  steel.  On  investigation  they  are  generally  found  to 
be  chiefly  mixtures  of  carbonaceous  and  cyanogen  compounds  possessing  the  well- 
known  carburizing  properties  of  those  substances. 

Mechanism  of  Cementation.  —  It  was  held  for  many  years  that  in  the  cementa- 
tion of  iron  solid  carbon  passed  bodily  from  the  packing  materiarinCo  the  metal,  fol- 
lowed hy  a  slow  migration  towards  the  center.  Recent  investigations,  however,  have 
made  it  evident  that  the  transfer  of  the  carbon  from  the  packing  material  to  the 
metal  is  accomplished  chiefly,  if  not  altogether,  by  means  of  some  gases  liberated  or 
formed  during  the  annealing  treatment.  It  has  been  shown  quite  conclusively,  for 
instance,  that  if  a  piece  of  steel  surrounded  by  pure  carbon  be  heated  in  vacuum,  thus 
precluding  the  formation  of  gases,  it  will  not  take  up  carbon,  although  one  observer 
has  noted  that  if  decided  pressure  be  applied  some  carbon  will  pass  into  the  iron  even 
in  the  absence  of  gases.  Whether  this  be  so  or  not  it  is  apparently  certain  that  the 
carbon  must  first  be  volatilized  before  becoming  very  active  as  a  carburizing  agent  in 
the  cementation  and  case  hardening  treatments. 

Carbon  monoxide  (CO)  and  volatilized  cyanogen  (CN)  compounds  are  the  gases 
which  seem  most  effective.  The  carbon  monoxide  is  derived  from  a  partial  combus- 
tion of  the  carbon  of  the  cementing  material  by  atmospheric  oxygen  while  the  cyan- 
ogen results  from  a  combination  of  that  carbon  with  atmospheric  nitrogen  or  from 
the  decomposition  of  cyanide  compounds  such,  for  instance,  as  potassium  cyanide 
and  ferro-cyanide.  It  may  be  assumed  that  the  carbon  monoxide  once  formed  gives 
up  its  carbon  to  the  iron  according  to  the  reaction, 

2CO  +  3Fe  =  Fe3C  +  CO2, 

the  resulting  Fe3C  or  cementite  being  dissolved  by  the  austenite  very  much  as  salt  is 
dissolved  in  water  and  the  CO2  being  again  reduced  to  CO  on  coming  in  contact  with 
fresh  carbon  (CO2  +  C  =  2CO).  The  marked  activity  of  cyanogen  compounds  com- 
pared t'o  the  slower  action  of  charcoal  have  led  some  to  believe  that  cyanogen  gases 
are  especially  effective  in  carburizing  iron.  It  should  be  noted,  however,  that  while 
cyanide  compounds  produce  a  much  quicker  carburization  they  soon  lose  their 
carburizing  power  so  that  when  deep  cases  are  needed,  as  in  the  manufacture  of 
blister  bars,  charcoal,  acting  chiefly  through  the  production  of  carbon  monoxide,  is 
preferable. 

Cooling  from  Carburizing  Temperature.  —  It  is  generally  desired  that  articles  sub- 
jected to  the  case  hardening  treatment  should  have  a  very  hard  surface.  To  produce 
this  hardness  the  case  hardened  articles  should  be  quenched  from  above  their  critical 
range.  The  prolonged  heating  at  a  very  high  temperature  to  which  these  articles 
have  been  exposed,  however,  has  developed  a  coarseness  of  structure  both  in  the 
core  and  in  the  case  which  would  be  retained  if  they  were,  as  they  sometimes  are, 
quenched  from  the  carburizing  temperature  or  after  cooling  to  a  somewhat  lower 
temperature.  It  is  obvious  that  in  order  to  impart  a  fine  structure  both  to  the  core 
and  to  the  case  the  articles  should  be  cooled  and  then  subjected  to  suitable  heat 
treatments. 


6         LESSON  XVI  — THE  CEMENTATION  AND  CASE  HARDENING  OF  STEEL 

Heat  Treatment  of  Case  Hardened  Articles.  —  In  order  to  refine  the  structure  of 
the  core  which  has  been  coarsened  by  a  long  exposure  to  a  high  temperature  the 
metal  should  be  reheated  slightly  above  the  critical  range  of  that  core  and  since  its 
carbon  content  seldom  exceeds  0.15  per  cent  carbon  a  temperature  of  at  least  900 
deg.  C.  should  be  used.  Guillet  recommends  1000  to  1025  deg.  The  finer  structure 
thus  imparted  to  the  core  will  then  be  retained  most  effectively  by  quenching  the 
metal  in  water  or  oil.  By  such  treatment,  however,  the  case,  although  hardened,  is 
still  relatively  coarse  since  its  quenching  was  effected  at  a  temperature  considerably 
exceeding  its  critical  range.  In  order  to  refine  it  while  leaving  the  structure  of  the 
core  undisturbed  the  article  should  now  be  reheated  slightly  above  the  critical  range 
of  the  case,  that  is,  to  some  750  or  800  deg.  C.,  and  then  quenched  in  oil  or  water.  By 
this  double  treatment  we  have  hardened  the  case  while  conferring  to  it  as  well  as  to 
the  core  a  fine  structure. 

Tempering  Case  Hardened  Steel.  —  It  has  been  seen  that  hardened  high  carbon 
steel  is  generally  subjected  to  a  tempering  process,  i.e.  reheated  to  some  200  or  300 
deg.  C.  in  order  to  decrease  its  brittleness  while  losing  but  little  hardness. 

There  seems  to  be  at  first  sight  no  apparent  reason  why  case  hardened  articles 
could  not  be  likewise  improved  by  suitable  tempering  following  the  hardening  of 
the  case.  On  second  thought,  however,  it  will  be  realized  that  since  the  chief  pur- 
pose of  tempering  is  to  toughen  the  hardened  steel  and  since  case  hardened  articles 
depend  for  their  toughness  on  the  toughness  of  their  cores  little  is  to  be  gained  by 
tempering  them. 

Experiments 

A  sample  representing  the  cross  section  of  a  case  hardened  bar  or  other  steel  object 
should  be  polished,  etched,  and  microscopically  examined.  The  structures  of  the  case 
and  core  described  in  the  lesson  should  be  noted. 

If  the  bar  has  not  been  retreated  after  cooling  from  the  carburizing  heat  it  should 
be  subjected  to  the  two  heat  treatments  described,  namely,  (1)  reheating  to  950  deg. 
C.  followed  by  quenching  in  water  or  oil  and  (2)  reheating  to  800  deg.  followed  by 
quenching  in  water.  A  transverse  section  should  be  prepared  for  microscopical 
examination  and  the  refining  of  both  the.  core  and  case  noted. 

Photomicrographs  of  the  various  structures  should  be  taken,  for  which  a  magnifi- 
cation of  50  to  100  diameters  will  be  sufficient. 


Examination 

I.     Describe  the  absorption  of  carbon  by  iron  above  its  critical  range. 
II.     Describe  the  best  treatments  to  be  applied  to  case  hardened  articles  after  car- 
burizing and  explain  why  they  are  needed. 


LESSON  XVII 

SPECIAL  STEELS 
GENERAL  CONSIDERATIONS 

The  steels  so  far  considered  in  these  lessons  are  the  ordinary  steels  of  commerce,  at 
present  often  called  "carbon"  steels  to  distinguish  them  from  the  "special"  steels 
of  relatively  recent  origin  but  of  rapidly  growing  importance.  By  special  steels  is 
meant  those  steels  which  owe  their  properties  in  a  marked  degree  to  the  presence  of 
one  or  more  special  elements  whereas  the  properties  of  carbon  steels  depend  chiefly, 
if  not  exclusively,  for  like  treatment,  upon  the  proportion  of  carbon  present.  Special 
steels  containing  but  one  special  element  are  commonly  called  "ternary"  steels, 
being  considered  to  be  made  up  of  three  constituents,  namely  iron,  carbon,  and  the 
special  element,  while  steels  containing  two  special  elements  are  called  "  quarternary  " 
steels  because  of  the  presence  of  four  constituents:  iron,  carbon,  and  the  two  special 
elements.  These  two  classes  of  special  steels  will  be  considered  separately. 

Ternary  Steels.  —  We  are  indebted  to  Guillet  for  a  brilliantly  conceived  and 
vigorously  developed  theory  of  the  ternary  steels.  Too  rigorous  an  application  of  the 
theory,  however,  should  not  be  insisted  upon  for  there  are  some  facts  not  yet  satis- 
factorily explained  by  it.  Its  use,  nevertheless,  will  be  found  an  invaluable  guide 
in  directing  researches  dealing  with  the  manufacture  and  the  application  of  these 
steels. 

Guillet's  theory  of  the  structure  and  properties  of  ternary  steels  may  be  briefly 
formulated  by  a  few  propositions.    It  is  also  represented  graphically  in  Figure  1. 

(1)  On  the  introduction  of  a  special  element  in  carbon  steel  the  latter  remains  at 
first  pearlitic,  but  as  the  proportion  of  the  special  element  increases,  the  carbon  re- 
maining constant,  it  becomes  first  martensitic  and  then  austenitic  (polyhedric),  as 
shown  graphically  in  Figure  1,  and  sometimes  cementitic  (carbide  steel)1  as  later 
explained. 

(2)  By  increasing  the  amount  of  carbon  present  in  a  special  steel,  the  proportion 
of  the  special  element  being  kept  constant,  it  is  generally  converted  from  a  pearlitic 
into  a  martensitic  condition  or,  if  already  martensitic,  into  an  austenitic  condition. 

(3)  The  greater  the  amount  of  carbon  the  smaller  the  proportion  of  the  special 
element  needed  to  cause  a  structural  transformation,  as  for  instance  pearlite  into  mar- 
tensite  or  martensite  into  austenite.    This  is  indicated  in  Figure  1. 

(4)  The  greater  the  amount  of  the  special  element  the  smaller  the  proportion  of 
carbon  needed  to  cause  a  structural  transformation.    This  is  also  shown  in  Figure  1. 

1  Guillet  uses  the  term  polyhedric  to  designate  an  austenitic  structure  and  carbide  steel  (acier  a 
rnrhiire)  to  indicate  the  presence  of  ccmcntite  (generally  in  special  steels  a  double  carbide  of  iron  and 
the  special  element).  It  seems  to  the  author  that  the  terms  austenitic  and  cementitic  are  preferable 
because  they  suggest  unmistakably  the  nature  of  the  constituents.  Austenitic  steels  are  not  the  only 
ones  exhibiting  u  polyhedric  structure;  ferritic  (low  carbon)  steels  for  instance  are  also  polyhedric. 

1 


2  LESSON  XVII  — SPECIAL  STEELS 

(5)  No  very  sharp  lines  of  demarcation  are  observed  between  the  different  types 
of  structures  mentioned  in  the  preceding  propositions,  relatively  wide  ranges  of  com- 
position existing,  on  the  contrary,  in  which  the  steel  may  be  partly  pearlitic  and  partly 
martensitic  or  partly  martensitic  and  partly  austenitic,  etc.  These  transition  ranges 
are  indicated  by  shaded  areas  in  the  diagram  of  Figure  1.  Greater  refinement  in  the 
construction  of  this  diagram  would  undoubtedly  lead  to  the  introduction  of  a  troo- 
stitic  zone  between  the  pearlite  and  martensite  areas  and  possibly  also  of  a  sorbitic 
zone  between  pearlite  and  troostite. 

To  sum  up,  constituents  may  be  formed  during  the  slow  cooling  of  many  special 
steels  which  in  carbon  steels  can  only  be  produced  by  very  rapid  cooling  through  the 


.6  .3 

Percent    carbon. 


10 


Fig.  1.  —  Constitutional  diagram  of  special  steels. 

critical  range.  Carbon  steels,  moreover,  even  after  very  rapid  cooling  cannot  be  re- 
tained wholly  in  an  austenitic  condition  while  several  special  steels  remain  austenitic 
after  slow  cooling.  It  is  evident  from  the  above  and  from  the  diagram  that  in  order 
to  produce  a  certain  structure,  (1)  the  proportion  of  carbon  may  be  kept  constant 
while  the  proportion  of  the  special  element  is  increased  until  the  desired  structure  is 
obtained,  or  (2)  the  proportion  of  the  special  element  may  be  kept  constant  and  the 
proportion  of  carbon  increased,  or  (3)  both  the  proportion  of  carbon  and  of  the  special 
element  may  be  increased  when  the  desired  structure  will  be  obtained  more  quickly. 

The  usefulness  of  Quillet's  diagram  is  obvious.  Should  we  desire,  for  instance,  to 
know  the  kind  of  structure,  and  therefore  the  physical  properties,  of  a  steel  containing 
0.60  per  cent  carbon  and  8  per  cent  of  the  special  element,  the  diagram  shows  that- 
such  composition  falls  within  the  martensitic  range.  Likewise  a  steel  containing  one 
per  cent  carbon  and  15  per  cent  of  the  special  element  would  be  austenitic  according 
to  the  diagram.  Or  one  may  wish  to  know  what  proportion  of  the  special  element 


LESSON  XVII  — SPECIAL  STEELS  3 

should  be  added  to  a  carbon  steel  containing,  say,  0.5  per  cent  carbon,  to  make  it 
martensitic;  the  diagram  shows  that  7  per  cent  will  be  needed.  Again,  having  an 
austenitic  steel  containing  10  per  cent  of  the  special  element  it  may  be  desired  to  know 
the  minimum  amount  of  carbon  that  may  be  present  without  causing  the  steel  to 
become  martensitic;  the  diagram  shows  0.80  per  cent  of  carbon  to  be  the  smallest 
proportion  of  carbon  permissible. 

The  construction  of  such  diagrams  requires  the  preparation  of  a  number  of  alloys 
varying  in  their  contents  of  carbon  and  of  the  special  element,  their  microscopical 
examination  and  the  plotting  of  their  structure. 

It  is  quite  essential  to  know  the  rate  of  cooling  adopted  in  the  construction  of  the 
diagram,  i.e.  whether  the  samples  were  cooled  in  air  or  more  slowly  in  the  furnace, 
for  it  is  evident  that  their  structure  may  be  deeply  affected  by  thus  varying  the  speed 
at  which  they  cool.  Some  special  steels,  for  instance,  may  be  pearlitic  when  cooled 
very  slowly  in  the  furnace,  martensitic  when  cooled  in  air,  and  austenitic  after  water 
quenching. 

Influence  of  the  Special  Element  upon  the  Location  of  the  Critical  Range.  —  The 
production  of  martensitic  and  austenitic  structures  on  slow  cooling  is  due  to  the  fact 
that  the  special  element  lowers  the  position  of  the  critical  point  to  a  temperature  so 
low  (1)  as  to  permit  only  a  partial  transformation,  namely  of  austenite  into  marten- 
site,  the  steel  being  too  rigid  to  allow  a  more  complete  transformation,  or  (2)  as  to 
prevent  even  a  slight  transformation,  the  steel  in  that  case  remaining  austenitic. 
This  influence  of  the  special  element  in  lowering  the  position  of  the  critical  range  is 
depicted  in  Figure  2  in  which  it  is  assumed  that  the  proportion  of  carbon  remains 
constant.  It  has  been  further  arbitrarily  assumed  in  this  diagram  that  the  critical 
point  was  progressively  and  uniformly  lowered  from  700  deg.  C.  to  0  deg.,  as  the  pro- 
portion of  the  special  element  increased  from  0  to  6  per  cent.  From  many  observa- 
tions it  appears  (1)  that  as  long  as  the  critical  point  remains  above  300  deg.  C.  the  steel 
becomes  pearlitic  on  slow  cooling,  (2)  that  when  the  critical  point  is  lowered  below 
300  deg.  it  becomes  martensitic,  the  rigidity  of  the  metal  preventing  further  trans- 
formation, and  (3)  that  when  the  critical  point  is  lowered  to  atmospheric  temperature 
or  below  it  the  metal  remains  untransformed,  that  is,  austenitic.  These  inferences 
are  offered  here  because  of  their  apparent  usefulness  and  suggestiveness,  but  the  au- 
thor realizes  that  the  lines  indicating  the  relation  between  the  position  of  the  critical 
points  and  the  corresponding  structures  cannot  be  sharply  drawn,  for  they  are  likely 
to  shift  according  to  the  nature  of  the  special  element,  the  rate  of  cooling,  etc.  Again, 
troostitic  and  possibly  also  sorbitic  steel  are  likely  to  form  between  pearlite  and 
martensite,  that  is,  whenever  the  critical  point  is  lowered,  say  below  400  or  possibly 
below  500  deg. 

To  make  the  meaning  of  the  diagram  of  Figure  2  clear  let  us  consider  three  steels: 
I,  II,  and  III,  all  containing  one  per  cent  of  carbon,  but  respectively  1,  4.50,  and  7 
per  cent  of  the  special  element.  As  steel  I  cools  it  undergoes  its  transformation  at 
about  600  deg.  At  that  temperature  the  metal  is  so  plastic  that  the  transformation 
of  austenite  into  pearlite  readily  takes  place;  the  steel  becomes  pearlitic.  The  critical 
point  of  steel  II  is  slightly  below  200  deg.  At  this  temperature  the  transformation 
of  austenite  into  martensite  will  take  place,  but  the  metal  is  now  too  rigid  to  permit 
further  transformations;  the  steel  remains  martensitic.  Steels  which  remain  marten- 
sitic after  slow  (air)  cooling  are  said  to  be  "self-hardening."  In  the  case  of  steel 
III,  since  its  critical  point  is  lowered  below  atmospheric  temperature  it  necessarily  re- 


4  LESSON  XVII  — SPECIAL  STEELS 

mains  austenitic.  Since  austenitic  special  steels  have  their  points  of  transformation 
situated  below  atmospheric  temperature,  it  should  be  possible  through  cooling  to  a 
sufficiently  low  temperature,  as  for  instance  by  immersion  in  liquid  air,  to  cause  at 
least  their  partial  transformation,  that  is,  they  should  become  martensitic  after  such 
treatment  and  this  indeed  is  precisely  what  happens.  The  transformation  of  aus- 


floo. 


3  4  S 

°/0  Special    element. 
Carbon    I  % 

Fig.  2.  —  Influence  of  special  element  on  the  position  of  the  critical  point. 


tenite  into  martensite  takes  place  with  increased  volume  and  the  steel  from  non- 
magnetic becomes  magnetic. 

In  the  presence  of  a  special  element  lowering  the  critical  points  the  influence  of 
carbon  is  cumulative,  i.e.  the  greater  the  proportion  of  carbon  the  more  marked  the 
action  of  the  special  element.  An  attempt  has  been  made  in  Figure  3  to  show  this 
graphically.  The  diagram  indicates  the  position  of  the  critical  point  corresponding 
to  any  combination  of  carbon  content  between  0.25  and  1.50  and  of  the  special  ele- 
ment between  0  and  12  per  cent.  If  the  point  is  above  300  deg.  we  may  assume  that 
the  steel  is  pearlitic,  if  below  300  that  it  is  martensitic,  if  at  or  below  atmospheric 


LESSON   XVII  — SPECIAL  STEELS 


1 


j 


6  LESSON  XVII  — SPECIAL  STEELS 

temperature  that  it  is  austenitic.  In  this  illustration  arbitrary  values  have  been  given 
to  the  combined  influence  of  various  proportions  of  carbon  and  of  the  special  element 
upon  the  position  of  the  critical  points.  The  diagram  shows,  for  instance,  that  while 
with  0.50  per  cent  carbon  10  per  cent  of  the  special  element  are  required  to  lower  the 
critical  points  to  0  deg.  C.,  if  the  steel  contains  1.25  per  cent  carbon,  4  per  cent  of  the 
special  element  suffice.  The  production  of  pearlitic,  martensitic,  and  austenitic  struc- 
tures according  to  the  position  of  the  critical  point  has  also  been  indicated.  A  diagram 
of  this  kind  may  be  even  more  useful  than  Quillet's  for,  while  giving  the  same  kind  of 
information  as  his,  it  shows  in  addition  (1)  the  relation  between  the  composition  of  the 
.special  steel  and  the  position  of  the  critical  point  and  (2)  the  influence  of  the  position 
of  the  critical  point  upon  the  structure.  Its  construction  calls  for  many  determina- 
tions of  the  position  of  the  critical  point  in  steels  of  varying  composition  and  for  the 
microscopical  examination  of  the  corresponding  structures.  It  is  of  course  quite 
likely  that  as  experimentally  constructed  it  would  consist  of  more  or  less  smooth  curves 
rather  than  of  straight  lines. 

It  will  soon  be  shown  that  the  influence  of  special  elements  in  lowering  the  critical 
points  varies  greatly,  from  which  it  follows  that  some  elements  cause  the  production 
of  martensitic  and  austenitic  steels  much  more  readily  than  others.  Indeed  some 
elements  never  cause  a  sufficient  depression  of  the  critical  points  to  yield  austenitic 
or  even  martensitic  steels.  A  few  elements  even  actually  raise  the  location  of  the 
critical  points  in  which  case,  of  course,  the  steel  always  becomes  pearlitic  on  slow 
cooling,  regardless  of  its  composition. 

From  the  foregoing  considerations  it  appears  that  according  to  their  structural 
composition  special  steels  may  be  divided  into  at  least  four  classes,  (1)  pearlitic  steels, 
(2)  martensitic  steels,  (3)  austenitic  (polyhedric)  steels,  and  (4)  cementitic  (Quillet's 
carbide)  steels.  These  should  now  be  farther  considered. 

Pearlitic  Steels.  —  The  pearlitic  steels,  as  we  have  seen,  are  those  which  generally 
contain  but  a  relatively  small  amount  of  the  special  element,  although  in  case  of  steel 
very  low  in  carbon  the  proportion  of  the  special  element  may  be  quite  large.  In  these 
steels  the  special  element  may  (1)  be  dissolved  in  the  ferrite  forming  with  it  a  solid 
solution,  (2)  be  combined  with  carbon  in  cementite  as  a  double  carbide  of  iron  and  the 
special  element,  or  (3)  be  partly  dissolved  in  ferrite  and  partly  combined  with  carbon. 

According  to  Quillet  nickel  and  silicon,  for  instance,  are  entirely  dissolved  in 
ferrite,  while  manganese,  chromium,  tungsten,  vanadium,  and  molybdenum  are  partly 
held  in  solution  by  ferrite  and  partly  present  in  cementite  as  double  carbides.  Such 
terms  as  nickel-ferrite,  silico-ferrite,  mangano-ferrite,  etc.,  have  been  suggest  eel  to 
designate  ferrite  holding  in  solution  large  proportions  of  nickel,  silicon,  manganese, 
etc.,  respectively. 

The  structure  of  pearlitic  special  steels  is  generally  quite  similar  to  that  of  pearlitic 
carbon  steels,  although  the  pearlite  particles  of  special  steels  frequently  are  more 
angular  than  those  of  carbon  steel,  often  exhibiting  many  straight  sides  and  sharp 
corners,  whereas  the  pearlite  particles  of  carbon  steel  are  more  rounded.  The  lami- 
nation of  pearlite  is  also  often  more  minute  in  special  steels  while  for  same  carbon 
content  it  often  appears  to  occupy  a  larger  bulk.  From  their  similarity  of  struc- 
ture, it  might  reasonably  be  inferred  that  pearlitic  special  steels  should  not  differ 
much  in  physical  properties  from  ordinary  carbon  steels.  As  a  matter  of  fact,  however, 
pearlitic  special  steels  are  often  greatly  superior  to  carbon  steels  generally  because 
they  possess  in  a  much  greater  degree  that  desirable  combination  of  properties,  strength, 


LESSON  XVII  — SPECIAL  STEELS  7 

or  rather  high  elastic  limit,  and  ductility.  They  are  also  frequently  harder  for  like 
ductility,  and  therefore  better  adapted  to  resist  wear.  Finally  their  ability  to  resist 
shocks  is  often  markedly  superior  to  that  of  carbon  steels.  Their  uses  make  for  greater 
efficiency  and  their  greater  strength  permits  an  often  welcome  reduction  in  bulk  and 
weight  of  certain  parts  of  machinery.  This  greater  strength  and  stiffness  of  special 
pearlitic  steels  may  be  due  to  the  special  element  dissolving,  in  part  at  least,  in  the 
ferrite  thereby  increasing  its  strength,  elastic  limit,  and  hardness,  a  stronger  and  harder 
ferrite  resulting  in  turn  in  a  stronger  and  harder  pearlite.  The  superior  physical  qual- 
ities of  these  steels  may  also  be  due,  at  least  partly,  to  a  finer,  closer  ferrite-cementite 
aggregate. 

The  critical  points  of  special  pearlitic  steels  generally  occur  atrtemperatures  some- 
what lower  than  those  at  which  the  critical  points  of  carbon  steel  are  located,  this 
being  in  accord  with  the  usual  influence  of  special  elements  upon  these  points  as 
already  explained. 

Martensitic  Steels.  —  For  the  same  carbon  content  martensitic  steel  contains 
necessarily  more  of  the  special  element  than  pearlitic  steels,  while  for  a  given  propor- 
tion of  the  special  element  they  must  contain  more  carbon;  they  generally  contain 
both  more  carbon  and  more  of  the  special  element  than  pearlitic  steels.  As  already 
stated  the  influence  of  some  special  elements  in  lowering  the  critical  points  is  not  suffi- 
ciently pronounced  to  result  in  the  formation  of  martensite  on  slow  cooling.  Indeed 
some  elements  raise  the  position  of  the  critical  points  in  which  case  pearlitic  steel 
must  necessarily  always  be  formed  on  slow  cooling. 

The  properties  of  martensitic  special  steels  are  not  unlike  the  properties  of  marten- 
sitic carbon  steels,  that  is  of  hardened  carbon  steel.  These  steels  are  hard  and  brittle 
and  unforgeable  in  the  cold.  Their  uses  are  very  limited,  chiefly  because  of  their 
brittleness  and  of  the  difficulty  of  machining  them.  While  resembling  hardened  carbon 
steels  they  are  quite  stable  above  atmospheric  temperature,  being  little  affected  by 
tempering,  i.e.  by  reheating  to  200  or  300  deg.  C.  This  property  suggests  one  impor- 
tant application  at  least  of  martensitic  special  steels  later  to  be  considered,  namely, 
their  use  for  the  manufacture  of  cutting  tools,  their  greater  stability  permitting  the 
tools  to  be  heated  to  a  higher  temperature,  i.e.  the  cutting  being  performed  at  greater 
speed  without  breaking  down  through  excessive  tempering. 

The  martensite  of  special  steels  probably  is,  like  the  martensite  of  carbon  steels, 
chiefly  a  solid  solution  in  beta  iron  of  the  carbide  FesC  or  more  often  of  a  double  car- 
bide of  iron  and  of  the  special  element,  the  magnetism  of  the  metal  being  due  to  the 
presence  of  some  alpha  iron. 

Austenitic  (Polyhedric)  Steels.  —  For  a  given  carbon  content  austenitic  steels 
necessarily  contain  more  of  the  special  element  than  martensitic  steels,  while  for  a 
given  proportion  of  the  special  element  they  are  necessarily  more  highly  carburized. 
Austenitic  steels  generally  contain  both  more  carbon  and  more  of  the  special  element 
than  martensitic  steels.  Their  properties  are  as  might  be  expected  similar  to  those  of 
austenitic  carbon  steels,  that  is,  of  high  carbon  steels  cooled  extremely  quickly  from 
a  very  high  temperature.  Austenitic  steels  are  moderately  tenacious  but  very  ductile; 
they  have  a  low  elastic  limit  but  possess  a  remarkable  power  of  resisting  wear  by  abra- 
sion as  well  as  rupture  by  shocks.  Their  mineralogical  hardness,  however,  is  gener- 
ally inferior  to  that  of  martensitic  steels. 

Unlike  quenched  austenitic  carbon  steel  these  special  steels  are  stable  at  all  tem- 
peratures below  their  point  of  solidification  and  are  not  therefore  greatly  affected  by 


8  LESSON  XVII  — SPECIAL  STEELS 

heat  treatment  unless  of  a  protracted  nature.  They  should  be,  however,  free  from 
separated  carbides.  If  not  they  should  be  heated  to  a  high  temperature  so  as  to  cause 
the  solution  of  these  carbides,  and  then  quenched  to  prevent  their  separating  again. 

The  austenite  of  special  steels  undoubtedly  consists  of  a  solid  solution  in  gamma 
iron  of  carbon  and  of  the  special  element,  probably  of  a  double  carbide.  Because  of 
the  absence  of  alpha  iron  austenitic  steels  are  non-magnetic. 

Austenitic  special  steels  find  useful  application  for  parts  of  machinery  and  the  like 
subjected  to  very  severe  wear  by  abrasion  and  to  shocks.  Their  low  elastic  limit  and 
the  difficulty  of  machining  them  are  the  chief  reasons  preventing  their  wider  use. 

Cementitic  (Carbide)  Steels.  —  Some  special  elements  on  being  introduced  in 
increasing  proportions  fail  to  convert  the  metal  into  austenite,  free  particles  of  a  double 
carbide  of  iron  and  the  special  element  being  formed  instead  and  embedded  in  a  mar- 
tensitic,  troostitic,  sorbitic,  or  pearlitic  matrix.  Guillet  calls  these  "carbide"  steels. 
Such  elements  as  chromium,  tungsten,  molybdenum,  and  vanadium  when  present  in 
sufficient  quantity  produce  cementitic  steels.  The  most  valuable  property  of  these 
steels  is  their  power,  when  the  carbides  are  embedded  or  rather  dissolved  in  a  mar- 
tensitic  matrix,  of  retaining  their  hardness  when  heated  to  such  temperature  as 
would  readily  cause  the  softening  of  hardened  carbon  steel,  thus  permitting  their  use 
in  the  shape  of  tools  at  such  speed  as  to  cause  their  cutting  edges  to  become  visibly 
hot.  This  phenomenon  will  be  further  explained  when  describing  self-hardening  and 
high  speed  steels. 

Treatments  of  Special  Steels.  —  Special  steels  are  subjected  to  the  same  treat- 
ments as  carbon  steels,  i.e.  to  hot  and  cold  working,  annealing,  hardening  and  temper- 
ing, and  to  case  hardening.  Since  the  special  elements,  however,  often  have  a  marked 
influence  on  the  position  of  the  critical  points  it  is  obvious  that  the  temperatures  in- 
dicated as  the  most  suitable  ones  for  the  annealing  and  hardening  of  carbon  steels 
may  not  be  satisfactory  in  the  case  of  special  steels.  The  position  of  the  critical 
points  should  in  every  case  be  determined  and  the  heat  treatments  conducted  accord- 
ingly. Greater  care  is  also  frequently  needed  in  the  forging  of  special  steels,  many 
of  them  not  being  quite  as  malleable  as  carbon  steels.  Finally  some  of  the  special 
elements  promote  the  absorption  of  carbon  by  iron  below  its  solidification-point  while 
others  oppose  it  and  these  influences  must  be  considered  in  case  hardening  special 
steels.  The  treatment  of  special  steels  will  be  considered  further  in  the  next  lesson  in 
connection  with  the  description  of  some  of  the  most  important  commercial  types. 

Treatment  of  Pearlitic  Steels.  —  Pearlitic  special  steels  may,  like  carbon  steels, 
be  subjected  to  annealing,  hardening,  tempering,  and  case  hardening.  Their  critical 
points,  however,  being  generally  lower,  the  proper  temperatures  for  these  operations 
are  likewise  lower.  They  should  be  determined  for  each  steel.  According  to  Guillet, 
however,  the  steel  should  be  heated  quite  a  little  above  its  critical  range  because  in 
the  case  of  the  pearlite  of  special  steels  its  transformation  into  a  solid  solution  does  not 
take  place  as  readily.  The  steel  should  then  be  cooled  to  near  its  critical  range 
before  quenching.  Guillet  writes  that  pearlitic  special  steels  may  be  divided  into 
(1)  those  that  are  not  very  sensitive  to  annealing,  namely  nickel  and  silicon  steels, 
and  (2)  those  that  are  very  sensitive  to  annealing,  namely  manganese,  chrome,  van- 
adium, tungsten,  and  molybdenum  steels.  It  should  be  noted  that  in  the  first  group 
the  special  elements  are  supposed  to  be  entirely  dissolved  in  the  iron,  while  in  the 
second  group  they  are  partly  dissolved  and  partly  present  as  carbides.  The  case  harden- 
ing of  pearlitic  special  steels  may  result  in  the  production  of  martensitic  or  even  austen- 


LESSON   XVII  — SPECIAL  STEELS  9 

itic  cases  without  the  necessity  of  rapid  cooling  from  above  the  critical  range.  This 
will  be  readily  understood  by  referring  to  Figure  1  where  it  will  be  seen  that  by  keep- 
ing the  proportion  of  the  special  element  constant  and  increasing  the  carbon  the 
steel  may  be  converted  from  a  pearlitic  to  a  martensitic  and  even  to  an  austenitic 
condition.  The  nearer  the  steel  to  the  boundary  between  the  pearlitic  and  marten- 
sitic zones  the  more  readily,  of  course,  will  it  become  martensitic  on  case  hardening 
because  the  smaller  the  amount  of  carbon  needed  to  produce  that  transformation. 
This  possibility  of  producing  steel  objects  with  a  pearlitic  soft  core  and  a  hard  marten- 
sitic shell  without  quenching  from  a  high  temperature  and  therefore  without  exposing 
the  objects  to  the  dangers  of  the  quenching  bath  does  not  seem  to  have  received  the 
attention  it  deserves  for  it  suggests  important  practical  applications.  And  likewise 
the  production  of  a  soft  pearlitic  core  surrounded  by  a  hard  martensitic  steel,  itself 
surrounded  by  a  tough  austenitic  steel.  It  points  at  least  to  the  manufacture  of  pearl- 
itic steel  objects  which  can  be  readily  machined,  etc.,  and  as  a  last  treatment  made 
austenitic  to  a  certain  depth,  being  in  this  way  greatly  superior  to  the  austenitic  steels 
at  present  used,  which  being  cast  in  an  austenitic  condition  can  be  machined  only 
with  very  great  difficulty. 

Treatment  of  Martensitic  Steels.  —  Unlike  martensitic  carbon  steels  martensitic 
special  steels  being  quite  stable  below  the  critical  range  of  the  metal  are  not  readily 
affected  by  tempering  treatments.  It  should  be  borne  in  mind,  however,  that  some 
special  steels  which  are  martensitic  after  air  cooling  may  become  pearlitic,  in  part  at 
least,  after  very  slow  cooling  in  the  furnace  and  austenitic  or  martenso-austenitic 
after  quenching  in  water.  By  selecting  a  special  steel  of  suitable  composition,  for  in- 
stance, and  allowing  it  to  cool  in  a  furnace  it  becomes  pearlitic  and  can  in  consequence 
be  machined;  after  machining  the  finished  object  may  be  made  martensitic  by  cool- 
ing in  air,  doing  away  with  the  necessity  of  the  quenching  bath  and  its  inherent  evils. 
It  is  evident  that  for  this  purpose  the  composition  of  the  steel  should  be  near  the  bound- 
ary line  between  the  pearlitic  and  martensitic  zones.  The  author  believes  that  the 
practical  possibilities  of  this  procedure  have  been  overlooked.  When  working  near  the 
pearlito-martcnsitic  boundary  line  the  formation  of  troostite  is  of  course  always  likely. 

Treatment  of  Austenitic  Steels.  —  Austenitic  special  steels  are  stable  theoretic- 
ally at  least  at  all  temperatures  and  should  not,  therefore,  be  affected  by  heat  treat- 
ment of  any  kind.  Some  special  steels,  however,  may  require  air  cooling  to  be  truly 
austenitic,  in  which  case  very  slow  cooling  in  the  furnace  may  result  in  the  produc- 
tion of  some  martensite  or  troostite  and  even  of  some  pearlite,  accompanied  by 
the  reappearance  of  magnetism.  It  also  frequently  happens  that  during  the  rela- 
tively slow  cooling  of  austenitic  steels  some  free  cementite  may  be  formed,  consist- 
ing generally  of  a  double  carbide  of  iron  and  the  special  element,  this  setting  free  of 
cementite  being  generally  accompanied  by  a  decided  decrease  of  strength  and  ductil- 
ity. In  order  to  cause  the  reabsorption  of  the  separated  carbide  heating  to  a  high 
temperature  (1000  cleg.  C.  or  higher)  is  generally  required  followed  by  rapid  cooling 
in  water  or  oil  so  as  to  prevent  its  separating  again  on  cooling.  This  treatment  is 
sometimes  called  "  water  toughening." 

Treatment  of  Cementitic  Steels.  —  Cementitic  steels  contain  many  particles  of 
cementite  or  double  carbide  embedded  in  a  matrix  which  may  be  martensitic,  troo- 
stitic,  sorbitic,  or  pearlitic  according  to  the  rate  of  cooling.  It  is  often  desirable  to 
cause  the  disappearance  in  part  at  least  of  these  particles  while  producing  a  finely 
martensitic  structure,  and  for  this  purpose  heating  to  a  hisrh  *  ^vre  (1000  deg. 


10  LESSON  XVII  — SPECIAL  STEELS 

or  more)  followed  by  relatively  quick  cooling  is  necessary.  Cooling  in  air  is  often 
sufficiently  rapid  to  retain  the  carbide  in  solution,  as  for  instance  in  the  case  of  the 
high  speed  steels  soon  to  be  described. 

Quaternary  Steels.  —  Quaternary  steels  like  ternary  steels  may  be  pearlitic,  mar- 
tensitic,  austenitic,  or  cementitic  as  well  as  sorbitic  and  troostitic.  If  the  two  special 
elements  are  present  in  small  quantities  the  steels  remain  pearlitic.  If  they  contain 
one  or  two  cementite  forming  elements  such  as  chromium,  tungsten,  molybdenum,  etc., 
they  are  likely  to  be  cementitic,  that  is,  to  contain  many  particles  of  a  double  or  triple 
carbide.  These  should  generally  be  made  to  dissolve  in  the  matrix  by  heating  to  a 
high  temperature  followed  by  rapid  cooling  when  a  finely  martensitic  structure,  quite 
free  from  cementite,  may  be  produced  as  in  the  treatment  of  high  speed  steel.  If 
the  quaternary  steels  contain  considerable  proportions  of  two  special  elements  capable 
of  forming  solid  solutions  with  iron,  as  for  instance  nickel  and  manganese,  they  are 
frequently  martensitic  or  austenitic.  A  large  proportion  of  an  element  which  is  partly 
dissolved  in  ferrite  and  partly  present  in  cementite  as  a  double  carbide,  manganese, 
for  instance,  may  result  in  the  occurrence  of  cementite  particles  embedded  in  an 
austenitic  matrix. 

Examination 

I.  Describe  and  explain  Guillet's  diagram  showing  the  structural  composition  of 
special  steels  corresponding  to  varying  proportions  of  carbon  and  of  the  special 
element. 

II.     Explain  the  influence  of  some  special  elements  on  the  position  of  the  critical 
points. 

III.  Explain  the  superiority  of  pearlitic  special  steels  over  pearlitic  carbon  steels. 

IV.  Explain  any  difference  which  may  exist  between  the  martensite  of  special 

steels  formed  on  slow  cooling  and  the  martensite  of  carbon  steel  produced  by 
quick  cooling. 

V.     Explain  the  possible  production  in  the  case  hardening  of  some  pearlitic  special 
steels  of  martensitic  or  even  austenitic  cases  without  quenching. 


LESSON  XVIII 


SPECIAL  STEELS 

CONSTITUTION,   PROPERTIES,   TREATMENT,   AND.  USJSS_  OF  MOST 
IMPORTANT   TYPES 

The  present  lesson  is  devoted  to  a  brief  consideration  of  the  composition,  struc- 
ture, properties,  treatments,  and  uses  of  those  special  steels  which  have  been  found  to 
be  of  commercial  value,  namely,  nickel,  manganese,  tungsten,  chromium,  vanadium, 

JO 


Aus1~en  i  fie 


Martens  it  ic 


Pearl  iti  c 


O  0,4  0.8  1.2.  /.6 

%Carbon 

Fig.  1.  —  Nickel  steel.    Constitutional  diagram.    (Guillet.) 

silicon,  chrome-nickel,  chrome-vanadium,  chrome-tungsten,  and  chrome-molybdenum 
steels. 

Nickel  Steel.  —  Nickel  apparently  dissolves  in  iron  in  all  proportions.  The  con- 
stitutional diagram  of  nickel  steel  is  illustrated  in  Figure  1  after  Guillet.  In  view  of 
the  explanation  of  such  diagrams  given  in  the  preceding  lesson  it  will  be  readily  under- 
stood. It  shows  that  as  the  carbon  increases  from  0  to  1.60  per  cent  and  the  nickel 
from  0  to  30  per  cent  the  metal  which  at  first  remains  pearlitic  becomes  martensitic 

1 


LESSON  XVIII  —  SPECIAL  STEELS 


and  finally  austenitic.  The  nickel  steels  of  greatest  commercial  importance  seldom 
containing  more  than  5  per  cent  nickel  and  one  per  cent  carbon  are  pearlitic.  This 
influence  of  nickel  in  preventing  partly  or  wholly  the  transformation  of  austenite  into 
pearlite  is  due  to  its  lowering  the  critical  point  of  the  steel  as  fully  explained  in  the  last 
lesson  and  as  illustrated  graphically  in  Figure  2  in  the  case  of  iron-nickel  alloys  con- 
taining small  proportions  of  carbon.  It  should  be  remembered  that  the  presence  of 
larger  quantities  of  carbon  would  intensify  the  influence  of  nickel,  i.e.  would  cause 
the  critical  points  to  be  further  depressed.  The  diagram  shows  that  as  the  nickel  in- 
creases from  0  to  some  25  per  cent  both  transformations,  AB  on  cooling  and  A'B'  on 
heating,  are  depressed,  the  former,  however,  much  more  quickly  than  the  latter,  re- 
sulting in  a  rapidly  increasing  gap  between  the  two  transformations.  In  other  words, 
nickel  up  to  25  per  cent  greatly  increases  the  hysteresis.  Taking  a  steel,  for  instance, 
with  10  per  cent  nickel  cooling  from  a  high  temperature,  it  remains  non-magnetic 


A 
700 

600 
500 

«.oo 

300 
200 
100 


r 

0  ._5 


\ 


V 


SO         30         liO         SO         60         '0         80         90        IOQ 

Fig.  2.  —  Influence  of  nickel  on  the  critical  points  of 
iron.    (Osmond.) 

until  a  temperature  of  some  400  deg.  C.  is  reached  when  it  undergoes  the  magnetic 
and  other  transformations.  On  reheating  this  magnetic  steel,  however,  it  does  not 
again  lose  its  magnetism  until  a  temperature  of  some  675  deg.  is  attained.  Between 
400  and  675  deg.  this  nickel  steel  will  be  magnetic,  therefore,  in  case  its  last  transfor- 
mation resulted  from  cooling  below  400  deg.  and  it  will  be  non-magnetic  if  it  resulted 
from  heating  above  675  deg.  When  this  hysteresis  gap  between  the  transformation  is 
considerable  the  alloys  are  said  to  be  irreversible,  meaning  by  that  expression  that  the 
reverse  transformation  cannot  be  produced  at  or  near  the  same  temperature.  Nickel 
steels  containing  between  0  and  25  per  cent  are  therefore  often  spoken  of  as  irreversible 
alloys.  It  should  be  noted,  however,  that  when  the  nickel  content  does  not  exceed 
some  3  per  cent  the  alloys  are  really  reversible,  that  is,  the  gap  between  the  critical 
transformations  on  heating  and  cooling  is  not  excessive.  According  to  Osmond,  for 
instance,  with  3.82  per  cent  nickel  the  critical  point  on  heating  occurs  at  710  deg. 
and  on  cooling  at  628  deg.  A  gap  of  100  deg.  might  be  arbitrarily  selected  as  a  line 
of  demarcation  between  reversible  and  irreversible  alloys.  Returning  to  Figure  2  it 
will  be  seen  that  as  the  nickel  content  increases  above  25  per  cent  the  transformations 


LESSON   XVIII  —  SPECIAL   STEELS  3 

become  abruptly  reversible  (the  gap  between  them  not  exceeding  50  deg.),  that 
their  position  is  now  gradually  lifted,  reaching  a  maximum  for  about  70  per  cent 
nickel,  and  that  it  is  then  again  lowered.  Iron-nickel  alloys  containing  more  than  25 
per  cent  nickel  are  therefore  reversible. 

The  diagram  also  shows  that  with  some  25  per  cent  nickel  the  transformation  is 
lowered  below  atmospheric  temperature  which  means  that  the  metal  on  cooling  from 
above  B'  remains  non-magnetic  at  atmospheric  temperature  and  that  its  iron,  there- 
fore, is  in  the  gamma  condition  and  its  structure  austenitic. 

An  attempt  has  been  made  in  Figure  3  to  construct  a  diagram  indicating  the  re- 
lation existing  between  carbon  content,  nickel  content,  positiorrof-  the  critical  points 
on  cooling,  and  corresponding  types  of  structure  as  explained  in  Lesson  XVII. 


700 


75  Id.  125  15.  175  2O.  225          25. 


Nickel  %        O  23 


Fig.  3.  —  Influence  of  nickel  and  carbon  on  the  position  of  the  critical  point  An  and  corresponding 

types  of  structure. 

As  already  stated  the  pearlitic  nickel  steels  are  those  most  widely  used.  In  the 
majority  of  cases  the  nickel  content  does  not  exceed  3.50  per  cent  while  the  carbon 
content  is  seldom  over  0.50  per  cent.  These  steels  compared  with  carbon  steels  of 
equal  ductility  have  a  considerably  higher  strength  and  especially  higher  elastic  limit, 
while  compared  with  carbon  steels  of  like  elastic  limit  they  have  much  greater  duc- 
tility. To  explain  this  in  another  way,  the  introduction  of  some  3.50  per  cent  nickel 
in  a  0.50  per  cent  steel,  for  instance,  raises  its  elastic  limit  very  considerably  while 
decreasing  its  ductility  but  slightly.  Pearlitic  nickel  steels  are  also  somewhat  harder 
than  carbon  steels  of  like  properties,  hence  better  able  to  resist  wear.  When  properly 
heat  treated  their  ability  to  resist  shocks  is  likewise  greater. 

The  structure  of  pearlitic  nickel  steel  is  shown  in  Figure  4.  On  comparing  it  with 
that  of  carbon  steel  of  like  carbon  content  it  will  be  noted  that  the  pearlite  particles 
arc  somewhat  sharper  and  more  angular  and  the  fcrrite  grains  smaller.  When  exam- 


4  LESSON  XVIII  —  SPECIAL  STEELS 

ined  under  high  magnification  the  nickel  pearlite  is  seldom  found  as  distinctly  lami- 
nated as  ordinary  pearlite. 

The  hardening  and  annealing  of  nickel  steels  should  be  conducted  at  lower  temper- 
atures than  the  hardening  and  annealing  of  ordinary  steels  of  similar  carbon  content 
since  their  critical  points  occur  at  lower  temperatures.  From  the  evidences  at  hand 
it  would  seem  as  if  between  0  and  5  per  cent  nickel,  and  in  the  case  of  low  carbon 
steels,  each  one  per  cent  of  nickel  lowered  the  Ar!  point  some  20  deg.  C.  and  the  Aci 
point  some  10  deg.  In  the  nickel  pearlitic  steels  of  commerce,  therefore,  the  points 
Ari  and  Aci  should  occur  at  or  near  the  temperatures  indicated  in  the  following  table 
according  to  their  percentage  of  nickel. 


Fig.  4.  —  Nickel  steel.  Carbon  about  0.30  per  cent. 
Nickel  about  3  per  cent.  Magnified  100  diameters. 
(G.  A.  Reinhardt  in  the  author's  laboratory.) 


%  Ni 

Aci 

An 

0   

750  

700 

0.50  

745  

690 

1.00  

740  

680 

1.50  

.....  735  

670 

2.00  

730  

660 

2.50  

725  

650 

3.00  

720  

640 

3.50  

715  

630 

4.00  

710  

620 

4.50  

705  

610 

5.00  . 

.  700  . 

.  600 

While  nickel  retards  the  carburization  of  iron  by  case  hardening,  the  cores  of  nickel 
steel  articles  are  not  coarsened  by  the  high  temperature  of  the  carburizing  operation 
to  the  same  extent  as  carbon  steel  cores,  so  that  one  treatment  is  often  sufficient, 
namely  reheating  to  and  quenching  from  a  temperature  slightly  superior  to  the  crit- 


LESSON  XVIII  —  SPECIAL  STEELS  5 

ical  range  of  the  case,  that  is,  to  some  700  to  750  deg.  in  the  presence  of  some  3  or  3.5 
per  cent  nickel.  Higher  nickel  contents  call  for  lower  quenching  temperatures. 

The  case  hardening  of  nickel  steels  offers  the  possibility  already  alluded  to  of  pro- 
ducing a  martensitic  case  without  quenching.  Nickel  steel,  for  instance,  containing 
not  over  0.25  per  cent  carbon  and  some  3.50  or  more  per  cent  nickel  can  readily  be 
made  martensitic  near  the  outside  by  case  hardening  followed  by  air  cooling  as  shown 
in  Figures  5  and  6.  The  martensitic  grains  owe  their  polyhedral  form  to  the  original 
austenitic  grains  from  which  they  are  derived.  The  thickness  of  the  mastensitic  case 
is  about  0.5  mm.  The  occurrence  of  troostite  should  be  noted.  Under  lower  mag- 
nification (Fig.  6)  a  solid  troostite  band  is  seen  to  separate  the_martensitic  and  the 
sorbito-pearlitic  portions.  With  a  little  more  carbon  and  nickel  martenso-austenitic 
cases  may  be  produced  as  shown  in  Figures  7  and  8. 

Nickel  steels  that  are  martensitic  as  cast  are  not  utilized  because  like  all  mar- 
tensitic steels  they  are  hard,  brittle,  and  cannot  be  machined.  Their  case  hardening 
should  result  in  the  formation  of  ductile,  austenitic  cases.  Nickel  steels  which  are 
martensitic  after  air  cooling  may  be  troostitic,  sorbitic,  or  even  pearlitic  after  very 
slow  cooling  in  the  furnace,  while  they  may  become  austenitic  on  water  quenching. 
The  structure  of  martensitic  nickel  steel  is  shown  in  Figure  9. 

Austenitic  nickel  steels  are  not  widely  used,  the  high  carbon,  high  manganese 
steels  being  preferred  when  an  austenitic  steel  is  desired,  in  part  at  least  because  of 
their  lower  cost.  Like  all  austenitic  steels  they  are  non-magnetic,  ductile,  very  diffi- 
cult to  machine  and  have  a  low  elastic  limit.  Their  structure  is  polyhedric  (see  Fig.  10), 
Some  types  of  austenitic  nickel  steels  have,  however,  found  interesting  applications 
based  chiefly  on  the  marked  influence  of  nickel  on  the  dilatation  of  the  metal.  With 
36  per  cent  nickel,  for  instance,  the  dilatation  is  nearly  nil  and  the  resulting  alloy, 
discovered  by  Guillaume,  and  called  by  him  "invar"  is  used  successfully  for  the  con- 
struction of  clocks  and  other  instruments  of  precision.  With  some  46  per  cent  of 
nickel  and  0.15  per  cent  carbon  the  coefficient  of  dilatation  is  nearly  the  same  as  that 
of  glass  and  alloys  of  that  composition  called  "platinite"  are  used  in  place  of  plati- 
num for  the  construction  of  incandescent  electric  lamps.  Austenitic  nickel  steel,  like 
all  austenitic  special  steels,  may  be  made  martensitic  and  thereby  regain  its  magnetism 
by  immersion  in  liquid  air.  The  increase  of  volume  which  accompanies  this  trans- 
formation produces  a  swelling  of  the  polished  surface  which  because  of  the  resulting 
relief  effect  renders  the  structure  of  the  metal  apparent  without  etching,  as  shown  in 
Figure  11. 

Manganese  Steel.  —  Manganese,  when  alloyed  with  iron  and  carbon  in  large  pro- 
portion is  partly  dissolved  in  the  iron  and  partly  present  as  a  double  carbide  of  iron 
and  manganese.  From  this  behavior  of  manganese  the  structural  types  formed  by  in- 
creasing both  carbon  and  manganese  may  be  anticipated.  The  steel  should  at  first 
remain  pearlitic  and  then  become  in  succession  martensitic  and  austenitic.  With 
much  manganese  and  carbon,  however,  the  separation  of  carbide  is  to  be  expected. 
The  constitutional  diagram  of  manganese  steels  is  shown  in  Figure  12  after  Guillet, 
while  a  critical  point  structural  diagram  has  been  constructed  tentatively  in  Figure  13. 
By  comparing  the  constitutional  diagram  of  manganese  steel  with  that  of  nickel  steel 
it  will  be  noted  that  manganese  is,  roughly  stated,  twice  as  effective  as  nickel  in  pro- 
ducing a  certain  type  of  structure,  as  for  instance  in  converting  pearlitic  into  marten- 
sitic steel. 

Manganese  is  present  in  appreciable  quantities  in  all  ordinary  carbon  steels  but 


LESSON  XVIII  —  SPECIAL  STEELS 


Fig.  5.  —  Nickel  steel.  Nickel  3.44  per  cent.  Carbon  0.176  per  cent.  Case 
hardened  and  air  cooled.  Magnified  100  diameters.  (G.  A.  Reinhardt  in 
the  author's  laboratory.) 


Fig.  6.  —  Same  steel  as  in  Fig.  5.    Same  treatment.    Magnified  50  diameters. 
(G.  A.  Reinhardt  in  the  author's  laboratory.) 


LESSON   XVIII  — SPECIAL   STEELS 


Fig.  7.  —  Nickel  steel.    Nickel  4.86  per  cent.    Carbon  0.115  per  cent.     Case 
hardened  and  air  cooled.     Magnified  100  diameters.     (G.  A.  Reinhardt  in  the 

author's  laboratory.) 


—  Same  steel  as  in  Fig.  7.    Same  treatment.     Magnified  300  diameters. 
(G.  A.  Reinhardt  in  the  author's  laboratory.) 


8 


LESSON  XVIII  —  SPECIAL  STEELS 


unless  the  latter  contain  considerably  more  than  one  per  cent  of  that  element  they  are 
not  regarded  as  manganese  steels.  With  carbon  not  exceeding  0.80  per  cent  and  man- 
ganese not  exceeding  some  3  per  cent  the  steels  remain  pearlitic  and,  therefore,  not 
unlike  the  pearlitic  nickel  steels  so  widely  used.  Manganese  pearlitic  steels,  however, 


Fig.  9.  —  Nickel  steel.  Cast.  Nickel  10  per 
cent.  Carbon  0.80  per  cent.  Magnified  300 
diameters.  (Guillet.) 


Fig.  10.  —  Nickel  steel.  Nickel  25  per  cent. 
Carbon  0.80  per  cent.  Magnified  300 
diameters.  (Osmond.) 


a\ 

Fig.  11.  —  Nickel  steel.  Nickel  15  per 
cent.  Carbon  0.80  per  cent.  Cooled  in 
liquid  air  (-180  deg.  C.).  Not  etched. 
Magnified  300  diameters.  (Guillet.) 


are  practically  ignored  by  steel  manufacturers  and  users  apparently  (1)  because  of 
the  wide-spread  belief  that  such  steels  are  brittle,  and  (2)  because  of  the  difficulty  of 
manufacturing  low  carbon  manganese  steels.  The  belief  in  the  brittleness  of  pearlitic 
manganese  steels  is  founded  on  Hadfield's  statement  that  between  2  and  6  per  cent  of 
manganese  the  steels  are  hopelessly  brittle.  On  closer  examination,  however,  it  would 


LESSON   XVIII  —  SPECIAL   STEELS 


0)  /a 


c 

fc 


A  u  s  /e  n  i  f  i  c 


Martens/ 1 ic 


Pea  rl  iti  c 


0  0.4-  O.d  1.2. 

%  Carbon 

Fig.  12.  —  Manganese  steel.    Constitutional  diagram.    (Guillet.) 


700. 


60Q 


Fig.  13.  —  Influence  of  manganese  and 
carbon  on  position  of  critical  point  Ari 
and  corresponding  types  of  structure. 


10 


LESSON   XVIII  —  SPECIAL   STEELS 


seem  as  if  this  statement  was  true  only  in  the  case  of  rather  high  carbon  steels  cooled 
relatively  quickly.  Evidences  have  since  been  offered,  notably  by  Guillet,  showing 
that  low  carbon  pearlitic  manganese  steel  slowly  cooled  is  not  brittle.  These  steels  have 
been  difficult  to  manufacture  because  of  the  necessity  of  using  ferro-manganese  from 
the  blast  furnace  and  therefore  high  in  carbon,  thereby  introducing  much  carbon  in 
the  steel.  At  present,  however,  nearly  carbonless  ferro-manganese  is  produced  in 
electric  furnaces  and  also  by  the  thermit  process.  It  is  also  claimed  that  low  carbon 
manganese  steels  can  be  successfully  produced  in  the  electric  furnace  under  suitable 
oxidizing  conditions  and  at  high  temperature  when  carbon  may  be  oxidized  in  prefer- 
ence to  manganese.  If  the  physical  properties  of  low  carbon  pearlitic  manganese 
steel  are  at  all  comparable  to  those  of  pearlitic  nickel  steel  and  if  manganese  steel 
can  be  manufactured  more  cheaply  than  nickel  steel  of  like  properties  the  manufac- 


Fig.  14.  —  Manganese  steel.    Austenitic.    Cast.    Magnified  50  diameters. 


ture  and  testing  of  low  carbon  manganese  steel  should  receive  more  attention.  The 
possibility  of  producing  by  case  hardening  articles  having  pearlitic  cores  and  austen- 
itic  cases  also  deserves  some  consideration.  Martensitic  manganese  steels  are  not 
utilized  because  of  the  hardness  and  brittleness  which  they  share  with  all  martensitic 
steels.  Those  manganese  steels  whose  composition  is  near  the  boundary  line  between 
the  pearlitic  and  martensitic  regions  while  martensitic  after  air  cooling  may  be  troos- 
titic  or  even  pearlitic  after  very  slow  cooling  while  they  may  become  austenitic  on 
quenching. 

Austenitic  manganese  steel  is  of  considerable  industrial  importance.  It  is  often 
called  from  the  name  of  its  inventor  "  Hadfield  "  steel.  It  generally  contains  from 
10  to  15  per  cent  manganese  and  from  one  to  1.5  per  cent  carbon.  In  its  cast  condition 
it  is  weak  and  has  but  little  ductility  probably  because  of  the  presence  of  a  consider- 
able quantity  of  free  carbide.  On  being  heated  to  a  high  temperature  (1000  deg.  C. 
or  more),  however,  and  quenched  in  water  or  oil  its  tenacity  is  greatly  raised  and  it 


LESSON   XVIII  —  SPECIAL   STEELS 


11 


becomes  very  ductile;  the  treatment  being  often  called  on  that  account  "  water  tough- 
ening."   The  marked  change  of  properties  resulting  from  it  is  probably  due  to  the  ab- 


Fig.    15.  — Same   as    in   Fig.    14.      Magnified    300 
diameters. 


sorption  of  the  carbide  at  a  high  temperature  and  its  retention  in  solution  by  quick 
cooling.  The  structure  of  manganese  steel  both  in  its  cast  and  in  its  water  quenched 
condition  is  shown  in  Figures  14  to  16.  In  the  cast  sample  the  carbide  is  seen  to  occur 


Fig.  16.  —  Manganese  steel.  Austenitic. 
Water  quenched.  Magnified  200  diam- 
eters. (Guillet.) 


as  thick  membranes  surrounding  the  austenitic  grains  and  here  and  there  in  chunks. 
The  treated  sample  is  nearly  free  from  carbide  and  possesses  the  polyhedric  structure 
characteristic  of  gamma  iron  and  of  austenite.  The  properties  of  austenitic  man- 


12 


LESSON  XVIII  —  SPECIAL  STEELS 


ganese  steel  are  those  of  austenite,  namely  low  elastic  limit  but  great  hardness  and 
wearing  power  combined  with  much  ductility. 

Tungsten  Steels.  —  Tungsten  appears  to  raise  rather  than  lower  the  critical  points 
of  iron  while  it  forms  with  it  a  double  carbide  of  iron  and  tungsten  from  which  it  may 
be  safely  inferred  that  tungsten  steels  will  at  first  remain  pearlitic  on  slow  cooling 
and  that  as  the  percentage  of  tungsten  increases  it  will  become  cementitic,  that  is 
it  will  contain  carbide  particles.  This  is  shown  in  Figure  17,  which  is  a  reproduction 
of  the  constitutional  diagram  of  tungsten  steels  according  to  Guillet.  In  the  presence 
of  a  considerable  proportion  of  tungsten,  however,  the  position  of  the  Ari  point 

12 


d. 


Cemenf/ti  c 


Pearl  i tic 


04-  Q-Q 

°/o  Carbon 


I.G 


Fig.  17.  —  Tungsten  steel.     Constitutional  diagram.     (Guillet.) 

seems  to  be  greatly  affected  by  the  temperature  from  which  the  steel  cools.  Os- 
mond, for  instance,  found  in  the  case  of  a  steel  containing  0.42  per  cent  carbon  and 
6.25  per  cent  tungsten  that  heating  to  and  cooling  from  some  900  deg.  reveals  the 
existence  of  two  critical  points  respectively  at  690  and  650  deg.  As  the  temperature 
from  which  cooling  begins  increases,  however,  two  interesting  phenomena  are  ob- 
•  served,  (1)  the  upper  point  occurs  at  practically  the  same  temperature  but  becomes 
fainter  and  finally  disappears,  and  (2)  the  lower  point  remains  pronounced  but  its 
position  is  gradually  lowered.  Cooling  from  1015  deg.,  for  instance,  resulted  in  a  faint 
critical  point  at  670  deg.  while  the  lower  point  was  depressed  to  625  deg.  ;  cooling  from 
1210  deg.  caused  the  disappearance  of  the  upper  point  while  the  lower  remained  very 
pronounced  but  now  occurred  at  500  deg.  Bohler,  likewise  experimenting  with  a  steel 
containing  0.85  per  cent  carbon  and  7.78  per  cent  tungsten,  reports  the  existence  of  a 
point  at  710  deg.  and  one  at  550  deg.,  the  upper  one,  however,  occurring  only  when 


LESSON  XVIII  —  SPECIAL  STEELS 


13 


the  metal  has  not  been  heated  above  1 100  deg.  while  the  second  is  only  to  be  detected 
when  the  temperature  exceeds  1000  deg.  In  other  words  on  cooling  from  above 
1100  deg.  the  lower  point  only  occurs,  on  cooling  from  below  1000  deg.  only  the  upper 
point  is  visible,  while  heating  to  and  cooling  from  a  temperature  situated  between 
1000  and  1100  deg.  causes  the  appearance  of  both  points.  It  will  be  shown  soon  that 
this  indirect  influence  of  tungsten  upon  the  critical  points  affords  an  explanation  of 
the  remarkable  properties  of  self-hardening  and  high  speed  steels  which  are  chiefly 
tungsten  steels. 

On  heating  cementitic  tungsten  steels  to  a  high  temperature  the  particles  of  free 
carbides  are  dissolved  the  more  completely  the  higher  the  temperature.  Air  cooling 
is  often  sufficient  to  retain  the  carbides  in  solution  while  the  metal  becomes  finely 
martensitic.  Such  steels  are  said  to  be  "self-hardening."1  The  structure  of  two 


Fig.  18.  —  Tungsten  steel.  Tungsten  27.75 
per  cent.  Carbon  0.276  per  cent.  Mag- 
nified 200  diameters.  (Guillet.) 


Fig.  19.  —  Tungsten  steel.  Tungsten  39.96 
per  cent.  Carbon  0.867  per  cent.  Mag- 
nified 200  diameters.  (Guillet.) 


cementitic  tungsten  steels  is  reproduced  in  Figures  18  and  19.  The  white  particles 
are  the  double  carbide. 

Tungsten  steels  are  used  (1)  for  springs  generally  after  hardening  followed  by 
tempering,  (2)  for  magnets  after  hardening  only,  and  (3)  for  tools  as  self-hardening 
steels.  In  the  latter  case,  however,  considerable  manganese  is  always  present,  the 
resulting  alloy  being  in  reality  a  quaternary  steel.  High  speed  steels  are  quaternary 
steels  generally  containing  a  large  proportion  of  tungsten;  they  will  soon  be  described. 

Chrome  Steels.  —  Chromium  forms  a  double  carbide  with  iron  and  carbon,  while 
it  has  little  if  any  direct  influence  on  the  position  of  the  critical  points.2  The  presence 

1  The  presence  of  manganese  or  of  a  little  chromium  is  necessary,  however,  to  impart  self-hard- 
ening properties  to  tungsten  steel.    The  original  "self"  or  "air  hardening"  steel,  that  is  "Mushet" 
steel,  always  contained  considerably  more  than  one  per  cent  manganese  and  was  high  in  carbon. 

2  According  to  some  recent  observations  of  Nesselstrauss,  chromium  lowers  the  point  Ar3  of  hypo- 
eutectoid  steels,  eventually  causing  its  disappearance  while  it   raises  a  little  the  point  Ari.     The 
proportion  of  chromium  needed  to  cause  the  point  Ar3  to  disappear  is  the  smaller,  the  higher  the 
carbon  content.    With  0.20  per  cent  carbon,  5  per  cent  chromium  are  needed.    With  more  carbon  a 
correspondingly  smaller  percentage  of  chromium  suffices. 


14 


LESSON   XVIII  —  SPECIAL   STEELS 


of  chromium,  however,  like  that  of  tungsten  causes  the  point  on  cooling  to  be  markedly 
bwered  as  the  temperature  from  which  the  metal  cools  increases.  Osmond,  for  in- 
stance, found  the  following  relations  between  the  maximum  temperature  and  the  po- 
sition of  the  critical  point  on  cooling: 


MAXIMUM 
TEMPERATURE 

835 
1030 


CRITICAL  POINT 
ON  COOLING 

713-716 
682-692 


MAXIMUM 
TEMPERATURE 

1220 
1320 


CRITICAL  POINT 
ON  COOLINO 

635-643 
600-640 


o.Q  1.2, 

%  Carbon . 

Fig.  20.  —  Chrome  steel.    Constitutional  diagram.    (Guillet.) 


/.6 


The  constitutional  diagram  of  chrome  steels  is  shown  in  Figure  20  after  Guillet. 
Reheating  cementitic  chrome  steel  to  a  high  temperature  followed  by  quick  cooling 
(air  or  water)  results  generally  in  the  disappearance  of  some  of  the  particles  of  free 
carbide.  The  case  hardening  of  chrome  steels  yields  very  hard  cases.  As  the  core 
coarsens,  however,  these  steels  should  always  receive  the  double  treatment  described 
in  Lesson  XVI,  that  is,  (1)  reheating  and  quenching  for  refining  the  core,  and  (2)  re- 
heating and  quenching  for  refining  and  hardening  the  case. 

The  chrome  steels  that  are  utilized  seldom  contain  more  than  3  per  cent  chromium 
and  are  therefore  pearlitic  after  slow  cooling;  they  are  used  for  the  manufacture  of 
armor  piercing  projectiles,  of  steel  balls,  of  files,  and  of  some  other  tools,  the  presence 
of  chromium  increasing  the  hardness  and  the  hardening  power  of  the  metal. 

Becker  writes  that  the  hardness  imparted  by  chromium  is  not  accompanied  by  as 
much  brittleness  as  that  induced  by  carbon.  According  to  the  same  author  chromium 


LESSOX   XVIII  —  SPECIAL   STEELS 


15 


also  has  the  effect  of  increasing  the  elastic  limit  of  steel,  especially  when  it  is  com- 
bined with  vanadium.  The  structure  of  pearlitic  chromium  steel  resembles  that  of 
pearlitic  carbon  steel. 

Vanadium  Steels.  —  Vanadium  forms  double  carbides  with  iron  and  has  no  marked 
influence  on  the  positions  of  the  critical  points.  Unlike  tungsten  and  chromium  it 
dues  not  seem  to  cause  the  lowering  of  the  Ar  points  with  increasing  temperature. 
The  constitutional  diagram  of  vanadium  steels  is  shown  in  Figure  21  after  Guillet. 
Two  types  of  structures  are  produced,  pearlitic  and  cementitic.  Guillet  describes  the 
appearance  of  the  particles  of  free  carbide  as  being  triangular. 

According  to  Guillet  heating  cementitic  vanadium  steels  to  a  high  temperature 


Oeme n  // 1 1 


o 

o  o.s  i.o  1.5  2.0 

%Carbon 

Fig.  21.  —  Vanadium  steel.    Constitutional  diagram.    (Guillet.) 

fails  to  cause  the  absorption  of  the  free  carbide,  vanadium  steels  differing  in  this  re- 
spect from  other  steels  in  which  double  carbides  are  formed. 

The  vanadium  steels  commercially  utilized  seldom  contain  more  than  0.50  per 
cent  vanadium  and  are,  therefore,  pearlitic  and  free  from  carbides.  Their  proper- 
ties recall  those  of  pearlitic  nickel  steels,  namely  high  combination  of  elastic  limit  and 
ductility  and  high  resilience.  The  very  small  amount  of  vanadium  sufficient  to  pro- 
duce these  results  should  be  noted. 

Silicon  Steels.  —  Silicon,  probably  as  an  iron  silicide,  FeSi,  forms  a  solid  solution 
with  iron  in  all  proportions  and  has  no  very  marked  influence  upon  the  position  of  the 
critical  points  from  which  we  may  infer  that  slowly  cooled  silicon  steels  will  be  neither 
martensitic  nor  cementitic.  It  is  a  well-known  fact,  moreover,  that  silicon  has  a 
marked  tendency  to  cause  the  formation  of  graphitic  carbon  when  present  over  a 


16 


LESSON  XVIII  —  SPECIAL  STEELS 


certain  proportion  especially  in  high  carbon  steel.  The  constitutional  diagram  of 
silicon  steels  is  shown  in  Figure  22  according  to  Guillet.  It  will  be  seen  that  the 
structure  is  independent  of  the  carbon  content  being  entirely  regulated  by  the  pro- 
portion of  silicon.  As  long  as  the  proportion  of  silicon  does  not  exceed  5  per  cent  the 
steel  is  pearlitic  and  the  whole  of  the  carbon  remains  in  the  combined  condition. 
Between  5  and  7  per  cent  of  silicon  some  pearlite  is  still  present  and,  hence,  some  com- 
bined carbon,  but  graphitic  carbon  also  occurs;  between  7  and  20  per  cent  of  silicon 
the  whole  of  the  carbon  is  in  the  graphitic  condition,  the  balance  of  the  steel  consist- 
ing of  a  solid  solution  of  the  silicide  FeSi  in  iron  (silico-ferrite),  and  also,  according  to 

30 


^o. 


c 

o 
o 


10. 


o 


Graphite  +Fe  Si. 


Grap kite  +  s>oluf/on 


Pearl  ite+graphii~e  +  so  Sufi  on  Fe  3  i. 


Pearlfte+solufion 


0 


1.0  A5 

%  Carhon 

Fig.  22.  —  Silicon  steel.    Constitutional  diagram.     (Guillet.) 


20 


Guillet,  of  some  Fe2Si  likewise  dissolved  in  iron.  With  more  than  20  per  cent  of  sili- 
con the  steel  is  composed  of  graphite  and  of  FeSi. 

It  should  be  noted  that  the  only  silicon  steels  utilized  contain  less  than  5  per  cent 
of  silicon  and  are,  therefore,  pearlitic  and  free  from  graphitic  carbon  unless  indeed 
subjected  to  prolonged  annealing  or  very  slow  cooling  when  some  graphitic  carbon 
will  form,  especially  in  high  carbon  steels.  This  formation  of  graphitic  carbon  takes 
place  the  more  readily  the  higher  the  temperature,  the  longer  the  time  at  a  high  tem- 
perature, the  more  silicon  and  the  more  carbon  in  the  steel. 

Silicon  steels  are  chiefly  used  in  the  construction  of  dynamos  because  of  their 
low  magnetic  hysteresis  and  high  permeability  and,  with  the  addition  of  quite  a  little 
manganese,  for  springs  and  for  certain  parts  of  automobiles. 

Chrome-Nickel  Steels.  —  From  our  knowledge  of  the  constitution  of  nickel  steels 
and  of  chromium  steels  it  is  possible  to  foretell  the  constitution  of  the  quaternary 
chrome-nickel  steels.  Nickel  steels  being  pearlito-martenso-austenitic  and  chrome 


LESSON  XVIII  —  SPECIAL  STEELS  17 

steels  being  chiefly  pearlito-cementitic,  we  may  expect  that  in  chrome-nickel  steels, 
as  the  proportions  of  carbon,  nickel,  and  chromium  increase  the  steels,  at  first  pear- 
litic,  will  become  in  turn  martensitic,  austenitic,  and  cementitic.  In  the  case  of  ce- 
mentitic  steels  the  free  carbides  will  be  embedded  in  a  martensitic  or  austenitic  matrix 
according  to  their  composition.  It  should  also  be  expected  that  increasing  the  pro- 
portion of  nickel  (and  carbon)  will  cause  the  steel  to  pass  from  the  pearlitic  to  the 
martensitic  condition,  etc.,  while  increasing  the  chromium  (and  carbon)  will  make  it 
cementitic.  The  presence  of  both  nickel  and  chromium  in  the  same  steel  produces  a 
metal  possessing  the  valuable  qualities  of  both  nickel  and  chromium  steels,  namely, 
high  elastic  limit  combined  with  high  ductility,  greater  hardness^  hardening  power, 
resilience,  and  better  wearing  qualities  than  carbon  steels.  Chrome-nickel  steels  are 
especially  valuable  in  the  construction  of  parts  to  be  hardened  and  tempered  when 
they  yield  a  finely  martensitic  structure  having  greater  shock  resisting  power  than  the 
martensite  of  carbon  steels.  Practically  the  only  chrome-nickel  steels  utilized  are  the 
pearlitic  ones  containing  therefore  moderate  amounts  of  carbon,  nickel,  and  chro- 
mium. They  are  used  extensively  in  automobile  construction  and  for  the  manufacture 
of  armor  plates.  In  the  latter  case,  of  course,  one  face  of  the  plates  is  subjected  to 
the  case  hardening  treatment.  The  case  hardening  of  nickel-chromium  steel  resembles 
that  of  nickel  steel.  The  metal  should  be  reheated,  after  the  case  hardening  operation, 
slightly  above  the  critical  range  of  the  case  and  quenched.  Because  of  the  presence 
of  nickel  it  is  not  so  imperative  to  heat  to  and  quench  from  a  temperature  superior  to 
the  critical  range  of  the  core  before  hardening  the  case,  although  such  procedure  would 
probably  yield  a  tougher  core. 

Quaternary  Vanadium  Steels.  —  The  introduction  of  a  small  amount  of  vanadium 
into  the  various  special  steels  has  been  strongly  urged  and  nickel-vanadium,  chrome- 
vanadium,  chrome-nickel-vanadium  and  chrome-tungsten-vanadium  steels  have  been 
used.  It  is  claimed  that  the  presence  of  a  small  proportion  of  vanadium  (less  than 
0.50  per  cent)  increases  the  soundness  of  castings  and  their  freedom  from  occluded 
gases  and  that  it  adds  to  the  desirable  physical  qualities  of  forgings  such  as  strength, 
resilience,  ductility,  etc.  Guillet  writes  that  nickel-vanadium  steel  properly  hardened 
by  quenching  is  relatively  so  tough  that,  unlike  other  hardened  steels,  it  does  not 
require  tempering.  Since  vanadium  forms  a  double  carbide  with  iron  its  presence  in 
steel  is  likely  to  make  it  cementitic.  In  nickel  steel  and  in  nickel-chromium  steel  the 
matrix  will  be  pearlitic,  martensitic,  or  austenitic  in  accordance  with  the  proportion 
of  nickel  and  carbon  present;  in  chrome  steels  it  will  be  pearlitic  or  martensitic. 

Chrome-Tungsten  or  High  Speed  Steels.  —  Since  both  chromium  and  tungsten 
form  carbides  and  since  they  do  not  lower  the  Ari  points,  at  least  directly,  it  may 
be  fairly  anticipated  that  slowly  cooled  chromium-tungsten  steels  will  be  cementitic 
with  a  pearlitic,  sorbitic,  or  even  troostitic  matrix.  Upon  being  heated  to  a  high  tem- 
perature the  carbide  particles  are  dissolved  and  if  the  cooling  that  follows  be  suffi- 
ciently rapid  they  are  retained  in  solution,  the  metal  acquiring  a  finely  martensitic 
structure.  To  cause  a  complete  absorption  of  the  free  carbide,  however,  a  very  high 
temperature  is  often  required,  in  some  cases  approaching  the  melting  point  of  the  steel, 
while  air  cooling  is  frequently  sufficiently  rapid  to  prevent  the  carbide  from  again 
forming.  After  such  treatment  these  steels  although  fully  hardened  are  in  a  condition 
relatively  so  stable  that  they  may  be  heated  to  a  visibly  red  heat,  i.e.  to  some  600 
deg.  C.,  before  their  martensite  undergoes  any  marked  transformation.  This  invalu- 
able property  makes  it  possible,  with  tools  made  of  such  steels  suitably  treated,  to  cut 


18 


LESSON  XVIII  —  SPECIAL  STEELS 


steel  and  other  hard  metals  at  such  speed  that  the  cutting  edge  of  the  tool  becomes 
visibly  red  hot  before  breaking  down.  These  steels  are  known  in  consequence  as 
high  speed  steels.  Their  discovery  by  Taylor  and  White,  at  the  time  in  the  employ 
of  the  Bethlehem  Steel  Company,  South  Bethlehem,  Pennsylvania,  marks  one  of  the 
most  distinct  and  revolutionary  advances  ever  made  in  the  metallurgy  of  iron  and 
steel.  The  composition  of  these  steels  varies  greatly:  they  may  contain  from  0.25  to 
one  per  cent  carbon,  generally  not  over  0.60  per  cent;  from  5  to  25  per  cent  of  tungsten, 
generally  between  10  and  20  per  cent;  from  2  to  10  per  cent  of  chromium,  generally 
between  2  and  8  per  cent,  and  seldom  over  0.40  per  cent  of  manganese.  In  some  types 
tungsten  is  replaced  in  part  or  wholly  by  molybdenum;  in  others  a  small  amount  of 
molybdenum  is  present  in  addition  to  the  tungsten  and  chromium,  while  in  others  still 
a  small  amount  of  vanadium  (0.2  to  0.4  per  cent)  occurs.  Properly  treated  high  speed 


Fig.  23.  —  High  speed  steel.  Typical 
structure  after  correct  heat  treat- 
ment. Magnified  1000  diameters. 
(Edwards.) 


Fig.  24.  —  High  speed  steel.  Typical 
structure  after  annealing.  Magnified 
150  diameters.  (Edwards.) 


steels  often  exhibit  a  polyhedric  structure  (Fig.  23)  quite,  if  not  altogether,  free  from 
carbide  particles.  The  structure  of  a  similar  steel  annealed  is  reproduced  in  Figure  24. 

The  inventors  of  high  speed  steels  recommended  the  following  treatments  as  yield- 
ing the  best  results:  (1)  heating  the  tool  slowly  to  about  815  deg.  C.,  then  quickly 
until  its  extreme  edge  showed  indications  of  melting,  (2)  cooling  the  tool  quickly  to 
below  860  deg.  and  then  quickly  or  slowly  to  atmospheric  temperature,  and  (3)  re- 
heating the  tool  to  about  640  deg.  for  five  minutes  (in  a  lead  bath)  followed  by  cooling 
in  air.  The  author  believes,  however,  that  tools  of  high  speed  steel  are  now  generally 
heated  to  near  their  melting-point  followed  by  cooling  freely  in  air  or  in  an  air  blast, 
a  second  treatment  being  rarely  applied. 

The  remarkable  properties  of  high  speed  steels  briefly  outlined  in  the  foregoing 
pages  must  be  ascribed  to  the  formation  in  those  steels  of  a  martensitic  structure 
more  stable  than  that  of  quenched  high  carbon  steel  and  possibly  also  possessing  su- 
perior cutting  qualities.  An  explanation  of  the  greater  stability  and  better  quality 
of  the  martensite  of  high  speed  steels  is  suggested  at  least  by  the  well-known  indirect 
influence  of  chromium  and  tungsten  on  the  critical  points.  It  has  been  explained  that 
while  these  elements  have  no  marked  direct  influence  upon  the  location  of  the  critical 
points  their  presence  causes  the  critical  point  on  cooling  to  be  lowered  as  the  tempera- 


LESSON   XVIII  —  SPECIAL   STEELS 


19 


ture  from  which  cooling  begins,  increases.1  This  influence  is  shown  graphically  in 
Figure  25  in  which  the  line  AB  represents  the  maximum  temperatures  reached  before 
cooling  and  CD  the  position  of  the  critical  point  corresponding  to  these  temperatures. 
As  AB  ascends  CD  descends.  If  it  be  considered  that  martensite  forms  while  the  steel 
cools  through  its  critical  point  it  follows  that  as  the  steel  is  heated  to  higher  tempera- 
tures its  martensitic  structure  forms  at  gradually  decreasing  temperatures.  And  may 
we  not  conceive  that  the  lower  the  temperature  at  which  martensite  forms  the  greater 
its  stability  and  the  better  its  cutting  properties?  Might  not  its  superior  cutting  qual- 


/ooo. 


o 

I 

Ci 


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500. 


-4-00. 


A 


A/ 


Fig.  25.  —  High  speed  steel.     Relation  between  heating  temperature,   po- 
sition of  the  ATI  point,  and  the  stability  of  resulting  martensite. 

ity  be  due  to  its  having  been  formed  while  the  metal  was  cooler,  i.e.  stiffer  and  there- 
fore opposing  more  effectively  its  transformation?  It  should  be  more  stable  on  the 
ground  that  the  greater  the  range  of  temperature  between  the  temperature  of  a  con- 
stituent and  the  point  at  which  its  transformation  was  due  the  less  stable  its  condi- 
tion. In  Figure  25,  for  instance,  the  decreasing  distances  MN ',  M'N',  and  M"N" 
may  be  considered  as  proportional  to  the  instability  at  atmospheric  temperature  of 
the  martensitic  structures  resulting  from  cooling  respectively  from  L,  L',  and  L" 
and  formed  in  passing  respectively  through  M ,  M',  and  M" .  If  this  reasoning  be  cor- 
rect it  follows  that  the  lower  the  temperature  at  which  martensite  is  formed  the  greater 

1  This  was  shown  by  Osmond  several  years  before  the  introduction  of  high  speed  steel  and  it  is 
natural  to  infer  that  Taylor  and  White  were  guided  by  Osmond's  discovery. 


20  LESSON  XVIII  —  SPECIAL  STEELS 

must  be  its  stability  and,  therefore,  the  higher  the  temperature  to  which  it  may  be 
heated  before  undergoing  the  tempering  transformation  which  deprives  it  of  its  cutting 
properties. 

Edwards  believes  (1)  that  heating  high  speed  steel  to  1200  deg.  C.  results  in  the 
formation  of  a  carbide  of  tungsten  which  is  dissolved  by  the  iron,  (2)  that  the  critical 
point  then  exhibited  by  the  steel  at  about  380  deg.  is  not  the  point  Art  lowered  by 
the  presence  of  tungsten  and  chromium  but  that  it  marks  a  change  occurring  in  the 
carbide  of  tungsten,  (3)  that  heating  high  speed  steel  to  1320  deg.  C.  causes  the  for- 
mation of  a  double  carbide  of  tungsten  and  chromium  held  in  solution  by  the  iron, 
even  in  slow  cooling,  thus  explaining  the  absence  of  critical  point  below  900  deg.  in 
steel  so  treated,  and  (4)  that  the  failing  of  a  high  speed  tool  is  to  be  attributed  to  the 
formation  of  a  new  brittle  constituent  which  can  be  produced  by  reheating  the  steel  to 
700  deg. 

Experiments 

The  student  should  procure  some  samples  of  special  steels,  preferably  the  follow- 
ing: nickel  steel  containing  between  3  and  3.50  per  cent  nickel  and  between  0.25  and 
0.50  per  cent  carbon;  cast  manganese  steel  containing  between  10  and  15  per  cent 
manganese  and  between  one  and  1.5  per  cent  of  carbon;  chrome-tungsten  (high  speed) 
steel  of  good  commercial  quality. 

Nickel  Steel.  —  In  its  cast  or  forged  condition  the  nickel  steel  selected  is  pearlitic. 
Its  structure  should  be  normalized  by  heating  to  900  or  1000  deg.  C.  followed  by  slow 
cooling  and  examined.  The  differences  in  appearance  between  the  pearlite  particles 
and  those  of  ordinary  carbon  steel  subjected  to  like  treatment  (Lesson  V)  should  be 
noted.  Samples  of  this  steel  may  be  hardened  and  case  hardened  to  verify  the  ac- 
curacy of  the  statements  made  (1)  as  to  the  lower  temperature  needed  for  hardening, 
and  (2)  as  to  the  possibility  of  producing  martensitic  cases  without  quenching. 

Those  students  who  have  the  necessary  apparatus  are  advised  to  determine  the 
critical  points  of  this  steel  comparing  their  results  with  the  temperatures  indicated  in 
the  lesson. 

Manganese  Steel.  —  The  sample  of  manganese  steel  is  austenitic.  Its  structure 
in  the  cast  condition  should  be  examined  and  should  be  found  to  contain  many  car- 
bide particles,  possibly  forming  continuous  membranes  around  the  austenite  grains. 
A  sample  of  this  steel  should  be  heated  to  1000  or  1100  deg.  and  quenched  in  water. 
Its  structure  should  now  be  purely  austenitic  (polyhedric)  quite  if  not  altogether 
free  from  carbide  particles.  The  absence  of  any  critical  point  in  this  steel  should  be 
ascertained;  cooling  from  a  high  to  atmospheric  temperature  should  yield  a  smooth 
curve  free  from  heat  evolutions. 

High  Speed  Steel.  —  The  structure  of  this  steel  should  be  examined  both  in  the 
forged  condition  and  after  reheating  to  some  1200  deg.  followed  by  air  cooling.  The 
influence  of  tungsten  and  chromium  in  causing  the  lowering  of  the  critical  points  as 
the  temperature  of  cooling  increases  may  be  verified  by  those  having  the  necessary 
facilities. 

Photomicrographs  should  be  taken  from  all  structures  at  suitable  magnifications. 

Examination 

Describe  briefly  the  constitution,  properties,  treatments,  and  uses  of  (1)  nickel  steel, 
(2)  manganese  steel,  (3)  nickel-chromium  steel,  and  (4)  high  speed  steel. 


LESSON   XIX 

CAST  IRON 

Cast  iron  differs  from  steel  in  being  deprived  of  malleability.  —  This  lack  of  malle- 
ability is  due  to  the  presence  of  a  large  quantity  of  carbon,  generally  between  2.50  and 
4.00  per  cent.  The  carbon  present  in  cast  iron  may  be  (1)  wholly  in  the  graphitic 
condition,  (2)  wholly  in  the  combined  condition,  and  (3)  partly  in  the  graphitic  and 
partly  in  the  combined  condition.  These  three  types  of  cast  iron  should  be  separately 
considered.  The  factors  influencing  the  formation  of  graphite  or  of  combined  carbon 
should,  however,  first  be  recalled. 

Formation  of  Combined  and  Graphitic  Carbon.  —  It  will  be  explained  in  Lesson 
XXIII  that  when  cast  iron  solidifies  the  carbon  probably  remains  in  the  combined 
condition  and  that  the  resulting  carbide,  Fe3C,  is  partly  free  and  partly  in  solid  solu- 
tion in  the  iron.  This  FesC,  however,  is  an  unstable  compound  and  when  formed  at 
a  high  temperature  it  is  readily  decomposed  into  graphite  and  iron  according  to  the 
reaction 

'  Fe3C  =  3  Fe  +  C 

hence  the  formation  of  graphitic  carbon  in  cast  iron.  Two  factors  are  conspicuous 
in  promoting  the  formation  of  graphitic  carbon,  (1)  a  slow  rate  of  cooling  through  and 
below  the  solidification  period  and  (2)  the  presence  of  silicon.  The  gray,  i.e.  graphi- 
tic, cast  irons  are  generally  those  which  have  been  cast  in  sand  and  hence  slowly 
cooled  and  which  contain  a  relatively  large  percentage  of  silicon.  It  follows  that 
under  otherwise  identical  conditions  and  compositions  a  large  casting  will  become 
more  graphitic  on  solidifying  than  a  smaller  one  since  it  will  cool  more  slowly;  also 
that  of  two  castings  of  equal  size,  cooled  under  like  conditions  and  of  identical  com- 
position except  as  to  their  silicon  contents,  the  one  richer  in  silicon  will  contain  more 
graphitic  carbon.  The  conditions  most  effective  in  preventing  the  formation  of 
graphitic  carbon  and  in  promoting,  therefore,  the  retention  of  carbon  in  its  combined 
form  are  (1)  quick  rate  of  cooling  through  and  below  the  solidification  range  and  (2) 
the  presence  of  much  sulphur  or  manganese. 

Cast  Iron  Containing  only  Graphitic  Carbon.  —  Cast  irons  containing  a  consider- 
able amount  of  graphitic  carbon  are  known  as  gray  cast  irons  because  of  the  appear- 
ance of  their  fracture  which  is  grayish  or  blackish  and  coarsely  crystalline.  Cast 
irons  containing  the  whole  of  their  carbon  in  the  graphitic  condition  and  therefore 
free  from  combined  carbon  are  extreme  types  seldom  produced.  Their  structure, 
however,  should  be  considered. 

Proceeding  as  we  did  in  the  case  of  steel  we  shall  first  assume  cast  iron  to  be  a  pure 
alloy  of  iron  and  carbon,  free,  therefore,  from  its  usual  impurities.  If  the  whole  of 
the  carbon  is  in  the  graphitic  condition  it  is  evident  that  cast  iron  can  only  contain 
the  two  constituents  graphite  and  iron  or  ferrite.  We  may  therefore  anticipate  its 
structure.  It  will,  however,  be  interesting  to  study  the  mode  of  occurrence  of  the 

\ 


2  LESSON  XIX  — CAST  IRON 

graphitic  carbon.  The  structure  of  cast  iron  practically  free  from  combined  carbon 
is  illustrated  both  before  and  after  etching  in  Figures  1  and  2.  The  metal  will  be  seen 
to  consist  of  an  iron  or  ferrite  matrix  in  which  are  embedded  many  irregular  and 
generally  elongated  and  curved  plates  of  graphite.  These  graphite  plates  break  up 
so  effectively  the  continuity  of  the  metallic  mass  as  to  completely  destroy  the  ductility 
and  malleability  of  a  substance  (ferrite)  by  nature  very  ductile  and  malleable.  The 
brittleness  of  highly  graphitic  cast  iron  is  not  due  so  much  to  the  brittleness  of  the 
graphite  it  contains  nor  even  to  its  large  proportion  of  graphite  as  to  the  thorough 
manner  in  which  the  continuity  of  its  otherwise  ductile  matrix  is  destroyed  by  the 
shape  and  distribution  of  the  graphite  particles. 


Fig.  1.  —  Gray  cast  iron  free  from  combined  carbon.  Magnified 
100  diameters.  Not  etched.  (F.  C.  Langenberg  in  the  author's 
laboratory.) 

It  will  be  seen  in  another  lesson  that  when  the  graphite  occurs  in  small  rounded 
particles  as  it  does  in  malleable  cast  iron  the  ferrite  matrix  may  retain  considerable 
ductility  and  malleability. 

The  ferrite  matrix  of  this  highly  graphitic  cast  iron  (Fig.  2)  will  be  seen  to  be  made 
up  of  the  polyhedric  crystalline  grains  characteristic  of  carbonless  iron,  the  ferrite  of 
cast  iron  being  similar  in  this  and  other  respects  to  the  ferrite  of  wrought  iron  and  of 
hypo-eutectoid  steel.  In  impure  cast  iron  it  undoubtedly  holds  in  solution  silicon 
and  possibly,  to  some  extent,  other  impurities. 

Highly  graphitic  cast  iron  is  brittle  and  deprived  of  ductility  and  malleability 
because  of  the  presence  of  numerous  plates  of  graphitic  carbon;  it  is  weak  because  of 
the  presence  of  graphite  plates  and  because  of  the  relative  weakness  of  its  matrix;  it 
is  soft  and  therefore  easily  machined  because  of  the  softness  both  of  its  matrix  and  of 
the  graphite  it  contains;  it  expands  on  solidifying  because  of  the  formation,  with 


LESSON   XIX  — CAST   IRON  3 

increase  of  hulk,  during  solidification  of  a  large  amount  of  graphitic  carbon.  It 
should  be  noted  that  because  of  its  low  specific  gravity  graphite  will  occupy  a  rela- 
tively large  proportion  of  the  bulk  of  the  metal.  Cast  iron,  for  instance,  containing 
by  weight  3  per  cent  of  graphite  contains  by  volume  some  12  per  cent  of  that  element. 
As  might  lie  expected  the  rate  of  solidification  and  further  cooling  has  some  influ- 
ence both  upon  the  shape  and  size  of  the  graphite  particles  as  well  as  upon  the  size  of 
the  ferrite  grains,  and  therefore  upon  the  physical  properties  of  the  metal,  very  slow 
solidification  promoting  the  formation  of  large  graphite  plates  and  of  large  ferrite 
grains.  Were  it  possible  to  cause  the  graphite  in  cast  iron  to  occur  in  small  rounded 


Fig.  2.  —  Same  metal  as  in  Fig.  2.      Magnified  100  diameters 
Etched.     (F.  C.  Langenberg  in  the  author's  laboratory.) 

particles  instead  of  as  sharp,  curved  plates,  its  ductility  and  strength  would  undoubt- 
edly be  greatly  increased. 

The  diagram  of  Figure  3  shows  graphically  the  structural  composition  of  iron- 
carbon  alloys  in  which  the  whole  of  the  carbon  occurs  as  graphite.  Only  those  alloys, 
however,  containing  from  3  to  4.5  per  cent  carbon  can  be  produced.  Indeed  in  the 
absence  of  silicon  even  these  are  quite  unobtainable.  With  less  than  3  per  cent  car- 
bon it  is  well  nigh  impossible  to  prevent  the  retention  of  some  combined  carbon,  while 
with  less  than  2  or  at  least  with  less  than  1.50  per  cent  carbon  the  whole  of  the  carbon 
is  likely  to  be  in  the  combined  condition.  The  diagram,  therefore,  is  only  a  theoretical 
one.  It  has,  nevertheless,  its  interest  for  it  will  be  shown  in  another  lesson  to  repre- 
sent the  stable  and  final  equilibrium  of  the  iron-carbon  system.  The  percentage  of 
graphite  by  volume  has  also  been  indicated. 

Cast  Iron  Containing  only  Combined  Carbon.  —  Cast  iron  containing  only  com- 
bined carbon  and  free,  therefore,  from  graphitic  carbon  is  called  "white"  cast  iron 


4  LESSON  XIX  — CAST  IRON 

from  the  aspect  of  its  fracture  which  is  white,  brilliant,  and  highly  metallic.  The 
absence  of  graphitic  carbon  is  generally  due  (1)  to  the  presence  of  much  manganese 
and  sulphur  and  of  little  silicon,  (2)  to  quick  cooling  through  and  below  the  solidifica- 
tion period,  or  (3)  to  both  low  silicon,  high  manganese  and  sulphur,  and  quick  solidi- 
fication. Cast  iron,  for  instance,  may  contain  so  much  sulphur  and  manganese  and 
so  little  silicon  as  to  be  white  even  after  slow  solidification  or  it  may  solidify  so  quickly 
as  to  be  white  even  in  the  presence  of  much  silicon  and  little  manganese  and  sulphur. 
A  familiar  instance  of  the  marked  influence  of  the  rate  of  cooling  is  afforded  by  the 
casting  in  the  metal'molds  of  casting  machines  of  cast  iron  which  if  cast  in  sand  would 


Cast  /ron 
free  from 


IOO 


25. 


Graph  /Ye 
weighi". 


Ferr/fe. 


0~          ~/~  ~£~         ~J~  4 

Percent    Carbon. 

Fig.  3.  —  Structural  composition  diagram  of  iron-carbon  alloys  free  from  combined  carbon. 

have  been  gray,  whereas  it  is  now  white.     Small  castings  since  they  cool  more  quickly 
become  white  more  readily  than  larger  ones. 

In  the  absence  of  graphitic  carbon  the  structure  of  cast  iron  should  resemble  the 
structure  of  a  very  high  carbon  steel,  i.  e.  it  should  consist  after  slow  cooling  of  pearlite 
and  of  a  large  amount  of  free  cementite.  This  is  found  to  be  the  case  as  shown  in 
Figure  4  in  which  is  illustrated  the  structure  of  white  iron  containing  about  3  per  cent 
of  combined  carbon.  Theoretically  this  alloy  should  contain  nearly  63  per  cent  of 
pearlite  and  37  per  cent  of  free  cementite.  The  dark  constituent  in  the  photograph 
is  pearlite,  the  light  one,  visibly  in  relief,  cementite.  The  structure  of  white  cast 
iron  is  also  shown  in  Figure  5  under  high  magnification,  the  laminations  of  pearlite 
being  clearly  seen.  The  structure  of  white  cast  iron  is  further  illustrated  in  Figures 
6  and  7  after  Guillet.  These  photomicrographs  are  reproduced  here  because  they 


LESSON  XIX  — CAST   IRON  5 

afford  an  interesting  example  of  the  action  of  sodium  picrate  (Lesson  V,  page  7) 
in  coloring  free  cementite  while  leaving  pearlite  uncolored. 

The  structural  composition  of  white  cast  iron  is  to  be  calculated  like  the  composi- 
tion of  any  hyper-eutectoid  steel  of  known  carbon  content  as  explained  in  Lesson  V, 
the  following  relation  existing  between  the  percentage  of  pearlite  and  that  of  carbon 
in  the  iron,  on  the  assumption  that  pearlite  contains  0.834  per  cent  carbon: 


P  = 


800  -  120  C 


the  balance  of  the  metal  being  of  course  free  cementite.  While  -structurally  it  re- 
sembles high  carbon  steel,  white  cast  iron  is  deprived  of  malleability  being  indeed  very 
brittle  and  very  hard.  This  brittleness  and  hardness  are  due  to  the  very  large  pro- 
portion of  free  cementite  present  which  itself  is  very  hard  and  brittle. 


Fig.  4.  —  White  cast  iron.    Magnified  56  diameters. 


Fig.  5.  —  White    cast    iron.       Magnified 
500  diameters.    (Wtist.) 


It  will  be  evident  that,  starting  from  carbonless  iron,  as  the  carbon  increases  at 
first  low  carbon  steel  is  produced  and  then  in  succession  medium  high  carbon  steel, 
high  carbon  steel,  and  finally  white  cast  iron,  each  metal  passing  gradually  into  the 
next  without  any  sharp  line  of  demarcation  between  them.  It  is  logical  to  base  the 
distinction  between  high  carbon  steel  and  white  cast  iron  upon  the  malleability  of 
the  former  and  the  non-malleability  of  the  latter  and  this  is  altogether  a  question  of- 
carbon  content.  The  dividing  line  may  be  drawn  somewhat  arbitrarily  at  2  per  cent 
carbon.  As  a  matter  of  fact  steels  are  very  seldom  manufactured  containing  more 
than  1.75  per  cent  carbon  while  white  cast  iron  rarely  contains  less  than  2.25  per  cent 
carbon.  Between  the  steel  series,  therefore,  and  the  white  cast-iron  series  there  is  a 
natural  gap,  the  existence  of  which  generally  removes  any  doubt  as  to  the  nature  of 
the  metal  under  examination. 

Again  if  the  process  of  manufacture  be  known  there  need  be  no  doubt  as  to  the 
classification  of  any  highly  carburized  iron  alloy:  if  made  in  the  blast  furnace  from 


6 


LESSON  XIX  — CAST  IRON 


the  reduction  of  iron  ore,  it  is  cast  iron,  while  if  it  is  the  product  of  refining  cast  iron 
(by  the  Bessemer  or  the  open  hearth  processes),  or  of  the  remelting  under  oxidizing 
conditions  of  iron  or  steel  scrap  with  or  without  cast  iron  (open  hearth  process),  or  of 
the  carburizing  of  wrought  iron  (cementation  process),  or  of  the  carburizing  and  melt- 
ing of  wrought  iron  (crucible  process),  it  is  steel. 

The  diagram  of  Figure  8  indicates  graphically  the  structural  composition  both 
proximate  and  ultimate  of  any  iron  carbon  alloy  containing  from  0  to  6.67  per  cent 
combined  carbon,  that  is,  from  100  per  cent  ferrite  to  100  per  cent  cementite  assuming 
as  it  has  been  done  before  that  pearlite  contains  0.834  per  cent  carbon. 

It  will  be  noted  that  two  sources  of  ferrite  are  indicated  in  the  diagram,  namely, 
(1)  pearlite  (eutectoid)  ferrite  and  (2)  pro-eutectoid  or  free  ferrite,  the  sum  of  both 
being  known  as  total  ferrite,  while  four  sources  of  cementite  are  to  be  considered, 
namely,  (1)  pearlite  (eutectoid)  cementite,  (2)  pro-eutectoid  cementite,  (3)  eutectic 
cementite,  and  (4)  pro-eutectic  cementite,  the  sum  of  all  four  being  known  as  total 


«> 


«fe 


7* 


Fig.  6.  —  White  cast  iron.  Magnified  200  diam- 
eters.   Etched  with  picric  acid.    (Guillet.) 


Fig.  7.  —  .Sumo  metal  as  in  Fig.  6.  Magni- 
fied 200  diameters.  Etched  with  sodium 
picrate.  (Guillet.) 


cementite  and  that  of  (2),  (3),  and  (4)  as  free  cementite.  Let  us  recall  the  meaning 
of  these  terms: 

Pearlite  or  eutectoid  ferrite  is  the  ferrite  included  in  pearlite. 

Free  or  pro-eutectoid  ferrite  is  the  ferrite  liberated  as  hypo-eutectoid  steel  cools 
slowly  from  its  upper  critical  point  (Ar3  or  Ar3.2)  to  its  lower  point  (Art). 

Pearlite  or  eutectoid  cementite  is  the  cementite  included  in  pearlite. 

Pro-eutectoid  cementite  is  the  cementite  that  is  liberated  as  hyper-eutectoid  metal 
cools  slowly  from  its  upper  critical  point  (Arcm)  to  its  lower  point  (Ar3.2.i). 

Eutectic  cementite  is  the  cementite  included  in  the  austenite-cementite  eutectic 
which  forms  at  the  end  of  the  solidification  of  alloys  containing  more  than  some  1.70 
per  cent  carbon  as  explained  in  Lesson  XXIII. 

Pro-eutectic  cementite  is  the  cementite  which  forms  between  the  bcgmnin»:  ;  ml 
the  end  of  the  solidification  of  alloys  containing  more  than  4.3  per  cent  carbon  as 
explained  in  Lesson  XXIII.  Howe  calls  it  "primary"  cementite. 

The  portions  of  the  diagram  corresponding  respectively  to  the  steel  series  and  to 
the  white  cast-iron  series  have  also  been  indicated  leaving  two  groups  of  alloys  un- 


LESSON   XIX  — CAST   IRON 


Fig.  S.  —  Structural  composition  diagram  of  iron-carbon  alloys  free  from  graphitic  carbon. 


8 


LESSON  XIX  — CAST  IRON 


represented  by  industrial  products,  namely,  those  containing  between  1.70  per  cent 
and  2.25  per  cent  carbon  and  those  containing  more  than  5  per  cent  of  carbon.1  The 
steel  portion  of  this  diagram  has  been  used  in  Lesson  V. 

Cast  Iron  Containing  both  Combined  and  Graphitic  Carbon.  —  Cast-iron  castings 
nearly  always  contain  both  combined  and  graphitic  carbon.  In  the  majority  of  cases 
they  contain  from  0.25  to  1.50  per  cent  of  combined  carbon,  the  balance  of  that  ele- 
ment being  in  the  graphitic  condition.  The  chief  factors  affecting  this  distribution 
of  carbon  between  the  combined  and  the  graphitic  states  have  already  been  alluded 
to;  they  are  (1)  the  rate  of  cooling  during  and  below  solidification  (hence  the  size  of 
the  castings)  and  (2)  the  presence  of  silicon,  manganese,  and  sulphur,  the  first  element 
promoting,  and  the  last  two  opposing,  the  formation  of  graphitic  carbon.  If  it  be 


Fig.  9.  —  Gray  cast  iron.  Hypo-cutectoid  matrix 
(0.25  per  cent  combined  carbon).  Magnified  100 
diameters.  (Boylston.) 

considered  (1)  that  graphitic  carbon  is  soft,  (2)  that  the  presence  of  much  graphite, 
since  it  implies  little  combined  carbon,  means  the  occurrence  of  soft  ferrite  in  the  cast 
iron,  (3)  that  combined  carbon  (cementite)  is  very  hard,  and  (4)  as  later  explained, 
that  the  presence  of  combined  carbon,  at  least  up  to  a  certain  proportion,  greatly  in- 
creases the  strength  of  cast  iron,  it  will  be  evident  that  the  physical  properties  of  cast 
iron,  especially  its  strength  and  hardness,  will  depend  chiefly  upon  the  proportion  of 
combined  and  graphitic  carbon  it  contains. 

Let  us  first  consider  the  structure  of  cast  iron  containing  a  small  amount,tsay  0.25 

1  It  should  be  remembered  that  we  are  considering  here  pure  alloys  of  iron  and  carbon.  In  the 
presence  of  much  manganese  (or  chromium)  iron  may  contain  as  much  as  6.50  per  cent  and  even  much 
more  carbon,  while  in  their  absence  cast  iron  very  seldom  contains  more  than  4.5  per  cent  carbon. 
Again  some  unimportant  products  are  offered  for  sale  under  the  name  of  semi-steel  which  may  contain 
between  1.75  and  2.50  per  cent  of  total  carbon  entirely  combined,  forming,  therefore,  a  sort  of  connects 
ing  link  between  the  steel  and  the  cast-iron  series.  They  are  frequently  obtained  by  remelting  cast 
iron  in  cupola  furnaces  in  the  presence  of  considerable  iron  or  steel  scrap.  Washed  metal  likewise 
may  contain  between  1.75  and  2.50  per  cent  of  total  and  entirely  combined  carbon  but  this  is  a 
semi-finished  product  resulting  from  a  partial  refining  only  of  cast  iron. 


LESSON   XIX  — CAST   IRON  9 

per  cent,  of  combined  carbon  and  some  3  per  cent  of  graphitic  carbon  (Fig.  9).  The 
combined  carbon  will  yield  15  times  its  own  weight  of  cementite  (0.25  X  15  =  3.75 
per  cent)  and  the  resulting  cementite  eight  times  its  own  weight  of  pearlite 
(3.75  x  8  =  30  per  cent),  or  more  quickly, 

0.25  X  120  =  30  per  cent  pearlite 

exactly  as  in  the  case  of  steel. 

The  cast  iron  under  consideration  will  therefore  contain  30  per  cent  pearlite,  3 
per  cent  of  graphite,  and  the  balance,  67  per  cent,  necessarily  free  ferrite.  Cast  iron 
containing  graphitic  carbon  may  be  considered  as  being  made  up_oLtwo  distinct  parts, 
namely,  (1)  graphite  and  (2)  a  metallic  matrix  in  which  the  graphite  particles  are 
embedded.  It  will  be  obvious,  moreover,  that  the  metallic  matrix  of  gray  cast  iron 
is  in  reality  a  steel  matrix  since  it  necessarily  consists,  like  steel,  of  an  aggregate  of 


Fig.  10.  —  Gray  cast  iron.     Eutectoid  matrix.     Magni- 
fied 500  diameters.    (Boylston.) 

ferrite  and  cementite  partly  associated  to  form  pearlite.  In  cast  iron  free  from  com- 
bined carbon  the  matrix  is  pure  ferrite;  with  a  little  combined  carbon  it  is  of  the  nature 
of  a  low  carbon  steel;  as  the  combined  carbon  increases  the  metallic  matrix  is  con- 
verted into  steel  of  increasing  carbon  content;  with  less  than  some  0.80  per  cent  car- 
bon the  matrix  resembles  an  hypo-eutectoid  steel;  with  some  0.80  per  cent  carbon 
the  matrix  is  pure  pearlite,  i.e.  eutectoid  steel;  with  more  than  0.80  per  cent  carbon 
some  free  cementite  is  formed,  the  matrix  consisting  of  hyper-eutectoid  steel.  In 
other  words,  as  the  proportion  of  combined  carbon  increases,  structural  changes  take 
place  in  the  metallic  matrix  of  cast  iron  identical  to  those  observed  and  described  in 
the  case  of  steel.  The  structure  of  cast  iron  containing  increasing  proportion  of  com- 
bined carbon  is  illustrated  in  Figures  9  to  11. 

The  above  considerations  justify  us  in  considering  gray  cast  iron  as  steel,  that  is, 
as  an  aggregate  of  ferrite  and  cementite,  the  continuity  of  which  is  destroyed  by  the 
presence  of  numerous  graphite  plates,  the  enormous  difference  in  properties  between 


10 


LESSON  XIX  — CAST  IRON 


cast  iron  and  steel  being  due  solely  to  the  presence  of  this  graphitic  carbon  or  rather 
to  the  breaking  of  the  continuity  of  the  mass  which  it  implies.  To  illustrate  further, 
if  it  were  possible  to  remove  bodily  from  a  chunk  of  cast  iron  every  particle  of  graphite, 
its  strength  and  ductility  would  not  probably  be  greatly  increased  because  its  con- 
tinuity would  still  be  effectively  destroyed  by  the  empty  spaces  once  occupied  by 
graphite.  If  not  too  high  in  carbon,  however,  it  would  now  be  weldable  and  could  be 
forged  into  a  small  piece  of  steel,  or  it  could  be  remelted  and  cast  into  a  sound  steel 
casting. 

Mottled  Cast  Iron.  —  Cast  irons  are  sometimes  produced  that  are  partly  gray  and 
partly  white,  that  is  made  up  of  particles  containing  graphitic  carbon  and  of  particles 
free  from  graphite  —  their  structure  is  well  shown  in  Figure  12.  They  are  called 
"mottled"  because  of  the  appearance  of  their  fracture. 


Fig.  11.  —  Gray  cast  iron.  Hyper-eutec- 
toid  matrix.  Magnified  500  diameters. 
(Wust.) 


Fig.  12.  —  Mottled  cast  iron.     Magnified 
500  diameters.    (Wiist.) 


Structural  Composition  of  Cast  Iron.  —  The  structural  composition  of  any  cast 
iron  of  known  percentages  of  graphitic  and  combined  carbon  can  readily  be  calculated 
by  following  the  method  employed  in  the  case  of  steel  and  assuming  pearlite  to  con- 
tain 0.834  per  cent  carbon,  that  is,  exactly  one  part  by  weight  of  cementite  and  seven 
parts  of  ferrite.  It  should  be  noted,  however,  that  in  the  presence  of  graphite  it 
requires  a  smaller  proportion  of  carbon  to  convert  the  whole  matrix  into  pearlite  since 
there  is  less  iron  to  be  so  converted.  To  make  the  matter  clear  in  order  that  cast 
iron  may  be  free  from  both  excess  ferrite  and  excess  cementite  it  must  contain  ferrite 
and  cementite  in  the  exact  proportion  of  one  part  of  cementite  to  seven  of  ferrite.  If 
the  cast  iron  contains  G  per  cent  of  graphite  and  C  per  cent  of  combined  carbon 
forming  15C  per  cent  of  cementite  the  balance  of  the  metal,  100  -  G  -  15C,  will  be 
ferrite.  Consequently  if 

100  -  G  -  15C  =  7  x  15C  =  105C 
or  100  -  G  -  120C  =  0 

that  is,  if  the  proportion  of  ferrite  equals  seven  times  that  of  cementite  the  matrix 
will  contain  pearlite  only;  if  100  -  G  -  120C  is  greater  than  0  the  matrix  will  be  hypo- 
eutectoid;  if  100  —  G  —  120C  is  smaller  than  0  the  matrix  will  be  hyper-eutectoid. 


LESSON   XIX  — CAST   IRON  11 

In  the  presence  of  3  per  cent  graphite,  for  instance,  the  following  relation  will  indicate 
the  needed  percentage  of  combined  carbon  to  make  the  matrix  entirely  pearlitic: 

100  -  3  -  120C  =  0 

which  gives  for  C  very  nearly  0.80  per  cent. 

It  will  be  sufficiently  accurate  to  assume  in  every  case  that  gray  cast  iron  contain- 
ing less  than  0.80  per  cent  combined  carbon  has  an  hypo-eutectoid  matrix  while  cast 
iron  containing  more  combined  carbon  has  an  hyper-eutectoid  matrix. 

In  calculating  the  structural  composition  of  cast  iron  two  cases  then  should  be 
considered,  (1)  the  cast  iron  contains  less  than  0.80  per  cent  combined  carbon;  it  has 
an  hypo-eutectoid  matrix  and  (2)  it  contains  more  than  0.80  per  cent  combined  car- 
bon; it  has  an  hyper-eutectoid  matrix.  In  the  first  instance  we  have  the  following 
relations  between  the  percentage  of  graphitic  carbon,  G,  the  percentage  of  combined 
carbon,  C,  the  percentage  of  ferrite,  F,  and  the  percentage  of  pearlite,  P, 

(1)  F  +  P  +  G  =  100 

(2)  P  =  8  x  15C  =  120C 

If  C  =  0.50  per  cent,  for  instance,  and  G  =  3  per  cent,  the  iron  would  contain  60 
per  cent  pearlite,  37  per  cent  ferrite,  and  3  per  cent  graphite. 

If  the  iron  has  an  hyper-eutectoid  matrix,  that  is  if  it  contains  more  than  0.80 
per  cent  of  combined  carbon,  we  can  write  the  following  equations,  Cm  representing 
the  percentage  of  free  cementite, 

(1)  P  +  Cm  +  G  =  100 

(2)  P  =  fF  =  f  (100  -  G  -  15C) 

the  second  equation  expressing  the  fact  that  the  totality  of  the  ferrite  (100  —  G  —  15C) 
is  included  in  the  pearlite  and  that  the  percentage  of  pearlite  is  equal  to  f-  times  that 
of  ferrite.  If  cast  iron  contains  2  per  cent  of  graphite  and  1.25  per  cent  of  combined 
carbon,  for  instance,  the  foregoing  equations  indicate  the  following  structural  com- 
position: pearlite  90.60  per  cent,  free  cementite  7.40  per  cent,  and  graphite  2  per  cent. 

The  structural,  graphical  diagram  of  cast  iron  containing  both  combined  and 
graphitic  carbon  has  been  constructed  in  Figure  13  in  accordance  with  the  scheme 
followed  in  these  lessons.  It  is  assumed  in  this  diagram  that  the  total  carbon  re- 
mains constant  at  3.50  per  cent  and  that  the  amount  of  combined  carbon  increases 
from  0  to  3.50  per  cent,  in  this  way  including  the  two  extreme  cases  corresponding 
respectively  to  absence  of  combined  carbon  and  of  graphitic  carbon.  If  this  diagram 
be  compared  with  that  of  the  structural  composition  of  steel,  Lesson  V,  p.  13,  the 
steel  nature  of  the  metallic  matrix  of  cast  iron  will  be  apparent.  It  will  be  noted  that 
in  the  present  diagram  when  the  proportion  of  combined  carbon  exceeds  1.7  per  cent 
there  are  two  sources  of  free  cementite  indicated,  namely,  pro-eutectoid  cementite  and 
eutectic  cementite;  the  origin  of  the  latter  will  be  made  clear  in  Lesson  XXIII.  Both 
of  these  cementites  constitute  the  free  cementite  present  in  cast  iron,  containing  more 
than  1.7  per  cent  of  combined  carbon;  while  formed,  as  later  explained,  at  different 
periods  of  the  cooling,  they  appear  to  coagulate  together  and  cannot  be  distinguished 
from  each  other  under  the  microscope. 

Physical  Properties  of  Cast  Iron  vs.  its  Structural  Composition.  —  The  physical 
properties  of  cast  iron  must  necessarily  depend  to  a  very  great  extent  upon  the  prop- 
erties of  its  steel  matrix  from  which  it  follows  that  its  hardness  and  strength  will 
increase  with  increasing  combined  carbon,  the  hardness  indefinitely,  the  strength  up 


12 


LESSON  XIX  — CAST   IRON 


to  the  eutectoid  carbon  ratio.  It  is  evident,  therefore,  that  cast  iron  of  maximum 
strength  (1)  should  have  a  steel  matrix  of  maximum  strength,  i.e.  should  contain 
in  the  vicinity  of  0.80  per  cent  combined  carbon  and  (2)  should  contain  as  little 
graphitic  carbon  as  possible  since  every  graphite  particle  is  a  source  of  weakness; 
in  other  words,  the  nearer  cast  iron  approaches  a  steel  of  maximum  strength  the  greater 
will  be  its  strength.  After  having  secured  the  desired  amount  of  combined  carbon  to 
give  strength  it  is  evident  that  a  reduction  of  the  graphitic  carbon  must  mean  a  cor- 
responding reduction  of  the  total  carbon  in  cast  iron.  In  ordinary  cupola  practise  for 
the  production  of  cast-iron  castings,  however,  which  consists  in  remelting  pig  iron  of 


Cos  f  /ron 
W/fh  Hypo-eufecfoid 
matrix 


Cosf  /ron 

with  Hyper-eufecfo/d 
matrix 


Graphite  • 


/oo 


O 


s 

o 


'  Pear/ite 


Free 

yo-G</1t 

rerrtfe. 


Combined  C 
Grvphite     C 


0.50 

aoo 


/.oo 


z.oo 


200 
1.50 


2.JO 

/.oo. 


-500 

oso 


3.50 
O 


Fig.  13.  —  Structural  composition  diagram  of  iron-carbon  alloys  containing  a  constant  propor- 
tion of  total  carbon  (3.50  per  cent),  but  varying  percentages  of  combined  carbon  (from  ft  to  3.50 
per  cent.) 

suitable  composition,  the  proportion  of  total  carbon  is  difficult  to  control,  being  neces- 
sarily between  3  and  4  per  cent  and  we  must  depend  to  produce  strength  almost  alto- 
gether upon  the  retention  in  the  combined  condition  of  a  suitable  proportion  of  carbon. 
The  total  carbon  may  be  decreased,  however,  by  the  use  of  iron  and  steel  scrap  as  part 
of  the  burden  of  the  cupola  resulting  in  increased  strength,  for  same  percentage  of 
combined  carbon,  or  by  remelting  in  a  so-called  "air  furnace,"  i.e.  under  oxidizing  con- 
ditions when  part  of  the  carbon  is  burnt  out.  These  low  total  carbon,  and  therefore 
tenacious,  cast-iron  castings  are  sometimes  offered  for  sale  under  the  name  of  semi- 
steels,  a  practise  somewhat  misleading  for  they  are  not  steel  in  any  sense  of  the  word 
since  they  are  not  malleable,  have  very  little  ductility,  and  generally  contain  a  con- 
siderable amount  of  graphitic  carbon. 

If  soft  cast-iron  castings  are  desired  so  that  they  may  be  easily  machined  they 
should  contain  as  little  combined  carbon  as  possible.     In  the  presence  of  but  little 


LESSON  XIX  — CAST  IRON  13 

combined  carbon,  however,  the  iron  will  not  be  very  tenacious,  strength  and  softness 
being  antagonistic.  If  the  castings  are  to  be  hard  they  should  contain  much  combined 
carbon  and,  therefore,  little  graphite.  In  extreme  cases  they  will  be  free  from  graphite, 
when  their  hardness  will  be  very  great,  but  they  will  then  also  be  very  brittle.  In 
the  majority  of  cases  castings  are  wanted  soft  enough  to  be  easily  machined  and  at 
the  same  time  of  fair  strength.  This  combination  of  properties  is  evidently  to  be 
obtained  by  producing  a  matrix  corresponding  to  a  medium  high  carbon  steel,  i.e.  by 
causing  the  cast  iron  to  retain  some  0.30  to  0.60  per  cent  of  combined  carbon. 

The  percentage  of  combined  carbon  in  cast  iron  upon  which  its  physical  properties 
primarily  depend,  can  be  ascertained  more  quickly  and  readily_by  microscopical  ex- 
amination than  by  chemical  analysis  and  quite  as  accurately. 

Chilled  Cast-iron  Castings.  —  It  is  sometimes  desired  to  produce  cast-iron  cast- 
ings very  hard  near  their  outside  but  soft  and  relatively  tough  near  their  center.  This 
may  be  done  by  so  regulating  the  composition  and  solidification  as  to  prevent  the 
formation  of  graphite  in  those  portions  that  should  be  hard  while  allowing  it  to  form 
in  the  portions  that  should  be  soft.  The  means  generally  employed  consist  in  using 
iron  plates  for  those  parts  of  the  molds  corresponding  to  the  parts  of  the  castings  that 
are  to  be  hard  and  sand  for  the  other  parts,  the  quicker  solidification  and  further  cool- 
ing of  the  metal  coming  in  contact  with  the  iron  plates  causing  the  retention  of  much 
combined  carbon.  The  resulting  castings  are  known  as  "chilled"  castings.  Impor- 
tant instances  of  the  application  of  this  method  are  to  be  found  in  the  manufacture 
of  chilled  cast-iron  wheels  and  of  chilled  rolls.  It  will  be  evident  that  the  presence 
of  sulphur  and  manganese  in  the  cast  iron  should  promote  the  retention  of  combined 
carbon  on  quick  cooling  while  the  presence  of  silicon  and  of  large  percentages  of  total 
carbon  should  hinder  it.  The  chemical  composition  of  cast  iron  to  be  converted  into 
chilled  castings  should  therefore  be  carefully  regulated.  Microscopical  examination 
should  prove  of  much  value  in  examining  the  depth  and  quality  of  "chills." 

Examination 

I.     Describe  the  structure  of  cast  iron  containing  both  graphitic  and  combined  car- 
bon and  compare  it  to  that  of  steel. 

II.  Assuming  pearlite  to  contain  0.83  per  cent  carbon  what  will  be  the  structural 
composition  of  cast  iron  containing  3.25  per  cent  graphite  and  0.40  per  cent 
combined  carbon  and  of  cast  iron  containing  1.90  per  cent  graphite  and  1.60 
per  cent  combined  carbon? 

III.  Describe  with  explanation  what  should  be  the  structure  and  composition  (as 
far  as  carbon  is  concerned)  of  cast  iron  with  the  following  properties:  (1)  Very 
soft  but  weak,  (2)  very  hard  but  brittle,  (3)  very  strong  but  lacking  in  softness, 
and  (4)  combining  moderate  strength  with  moderate  softness. 


LESSON  XX 

IMPURITIES   IN  CAST  IRON 

In  the  preceding  lesson  cast  iron  has  been  considered  as  a  pttre- alloy  of  iron  and 
carbon,  but  like  steel,  commercial  cast  iron  always  contains  varying  proportions  of 
silicon,  manganese,  phosphorus,  and  sulphur,  and  we  should  now  examine  the  influence 
of  these  impurities  on  its  structure  and  consequently  on  its  properties. 

Silicon  in  Cast  Iron.  —  Cast  iron  seldom  contains  less  than  0.50  per  cent  silicon  and 
frequently  as  much  as  3  or  3.50  per  cent.  As  in  the  case  of  steel  this  silicon  probably 
combines  with  some  of  the  iron  to  form  the  silicide  of  iron,  FeSi,  which  is  then  dissolved 
in  the  balance  of  the  iron.  The  ferrite  of  cast  iron,  therefore,  always  holds  a  consider- 
able amount  of  silicon  or  rather  of  the  silicide  FeSi  in  solution.  It  has  been  seen  that 
silicon  produces  exactly  three  times  its  own  weight  of  FeSi;  cast  iron  with  2  per  cent 
of  silicon,  for  instance,  will  contain  6  per  cent  of  FeSi  dissolved  in  its  ferrite.  The 
influence  of  silicon  on  the  properties  of  cast  iron  is  very  important  chiefly  through  its 
promoting  the  formation  of  graphitic  carbon  and,  therefore,  increasing  the  softness 
and,  if  carried  too  far,  decreasing  the  strength  of  cast  iron.  It  is  why  foundrymen 
often  speak  of  silicon  as  a  "softener."  That  it  increases  fluidity  while  decreasing 
shrinkage  and  chill  is  also  well  known. 

The  occurrence  of  varying  amounts  of  silicon  in  cast  iron  cannot  be  detected  under 
the  microscope,  unless  indirectly  and  roughly  through  the  presence  of  more  or  less 
graphitic  carbon.  It  is  probably  true,  however,  that  under  otherwise  similar  con- 
ditions, the  more  silicon  present  in  ferrite  the  slower  the  etching  of  that  con- 
stituent. The  ferrite  of  cast  iron  with  hypo-eutectoid  matrix,  for  instance,  possibly 
because  of  its  greater  silicon  content,  often  remains  brilliant  after  the  usual  deep 
etching  treatment  which  would  color  decidedly  some  of  the  grains  of  the  ferrite  of 
wrought  iron  or  of  low  carbon  steel. 

Sulphur  in  Cast  Iron.  —  Cast-iron  castings  of  good  quality  should  not  contain 
more  than  0.1  per  cent  of  sulphur  while  but  a  trace  of  that  element  may  be  present. 
In  the  manufacture  of  chilled  castings,  however,  as  much  as  0.2  per  cent  sulphur  is 
sometimes  allowed.  It  has  been  explained  in  Lesson  VI  that  because  of  the  great 
affinity  of  manganese  for  sulphur  these  two  elements  readily  combine  to  form  the 
manganese  sulphide  MnS,  each  part  by  weight  of  sulphur  giving  rise  to  the  forma- 
tion approximately  of  2 1/2  parts  of  MnS.  Cast  iron  with  0.05  per  cent  sulphur,  for 
instance,  would  contain  about  0.125  per  cent  of  MnS.  As  in  the  case  of  steel  this 
sulphide  occurs  in  the  form  of  rounded  particles  of  a  dove  gray  or  slate  color  embedded 
in  the  metallic  matrix  (Fig.  1).  Should  there  not  be  enough  manganese  present  to 
combine  with  all  the  sulphur,  the  remaining  sulphur  would  unite  with  iron  to  form  the 
sulphide  FeS  which  would  occur  as  rounded  yellow  areas.  It  should  be  noted,  how- 
ever, that  since  it  requires  less  than  two  parts  by  weight  of  manganese  to  combine 
with  one  part  of  sulphur,  when  cast  iron  contains  twice  as  much  manganese  as  it  does 
sulphur  no  iron  sulphide  can  be  formed,  theoretically  at  least.  Since  it  is  seldom  that 

1 


2  LESSON  XX  —  IMPURITIES  IN  CAST  IRON 

cast  iron  does  not  contain  a  considerably  greater  proportion  of  manganese  the  occur- 
rence of  FeS  in  cast  iron  should  be  rare. 

The  influence  of  sulphur  in  opposing  the  formation  of  graphitic  carbon  has  already 
been  mentioned;  it  may  consequently  be  said  to  harden  cast  iron.  It  has  also  a  well- 
known  influence  in  increasing  the  depth  of  "chill"  in  solidifying  cast  iron  against  a 
metal  wall,  that  is  the  thickness  of  metal  free  from  graphitic  carbon  produced  by  the 
cooling  action  of  that  wall.  Its  other  influences  are  harmful  as  it  increases  shrinkage, 
causes  the  molten  metal  to  be  sluggish,  and  induces  unsoundness. 

Manganese  in  Cast  Iron.  —  Special  cast  irons  are  made,  known  as  spiegeleisen, 
ferro-manganese,  etc.,  containing  very  large  proportions  of  manganese,  but  in  ordi- 
nary castings  the  amount  of  manganese  seldom  exceeds  2  per  cent  and  may  be  as  low 
as  0.10  per  cent.  It  has  been  shown  in  Lesson  VI  that  when  manganese  is  present  in 


Fig.  1.  —  Partial!}'  malleablized  cast  iron.  Magnified  670  diameters.  Sul- 
phur about  0.2  per  cent,  manganese  0.50  per  cent.  (C.  C.  Buck,  Cor- 
respondence Course  student.) 

these  relatively  small  proportions  it  first  combines  with  sulphur  to  form  the  sulphide 
MnS,  and  then  with  carbon  to  form  the  carbide  Mn3C,  this  carbide  uniting  with  the 
carbide  Fe3C  to  form  cementite.  The  cementite  of  cast  iron,  therefore,  like  that  of 
steel  nearly  always  contains  some  Mn3C.  Since  iron  and  manganese,  however,  have 
nearly  the  same  atomic  weights  it  remains  true  that  to  obtain  the  percentage  of  cemen- 
tite in  any  commercial  iron-carbon  alloy  it  suffices  to  multiply  its  percentage  of  com- 
bined carbon  by  fifteen. 

The  influence  of  manganese  in  opposing  the  formation  of  graphitic  carbon  has 
already  been  noted;  like  sulphur  it  is  a  hardener,  its  presence  in  large  proportions  in- 
creasing the  difficulty  of  machining  castings.  It  promotes  the  absorption  of  carbon 
by  iron.  Some  believe  that  it  increases  shrinkage  and  that  while  it  has  no  marked 
influence  on  the  depth  of  the  chill  it  increases  its  hardness. 

Phosphorus  in  Cast  Iron.  —  It  has  been  explained  in  Lesson  VI  that  when  phos- 
phorus occurs  in  very  small  quantities  as  it  does  in  steel,  the  totality  of  it  probably 


LESSON   XX  —  IMPURITIES   IN   CAST   IRON  3 

forms  the  phosphide  Fe3P,  which  is  then  dissolved  by  the  iron.  In  cast  iron,  how- 
ever, because  of  the  frequent  presence  of  a  considerable  proportion  of  phosphorus 
and  of  a  larger  proportion  of  carbon  this  element  assumes  another  condition.  The 
occurrence  of  phosphorus  in  cast  iron  was  first  studied  and  described  by  Stead.  The 
results  of  his  important  investigations  are  briefly  summarized  below: 

(1)  When  phosphorus  is  alloyed  with  carbonless  iron  in  amount  varying  from 
traces  to  1.70  per  cent,  it  forms  a  phosphide  corresponding  to  the  formula  Fe3P, 
which  is  held  in  solid  solution  by  the  iron.     All  the  metals  used  commercially,  such  as 
wrought  iron  and  steels  containing  very  little  carbon,  may  be  included  in  this  class. 
They  consist  essentially  of  polyhedric  grains  of  ferrite  holding  Fe3P  in  solution. 

(2)  When  the  metal  contains  from  1.70  to  10.2  per  cent  phospliorus  it  consists  of 
a  saturated  solution  of  Fe3P  in  iron  (1.70  per  cent  P)  and  of  a  eutectic  alloy  containing 
about  10.2  per  cent  P  and  made  up  of  about  61  per  cent  Fe3P  and  39  per  cent  of  the 


Fig.  2.  —  Alloy  of  iron  and  phosphorus.  Phos- 
phorus 1.8  per  cent.  Magnified  350  diam- 
eters. (Stead.) 


Fig.  3.  —  Alloy  of  iron  and  phosphorus.  Phos- 
phorus 8  per  cent.  Magnified  250  diameters. 
(Stead.) 


saturated  solution  of  Fe3P  in  iron.  To  account  readily  for  this  structure  and  that  of 
the  following  groups,  it  is  only  necessary  to  consider  these  metals  as  alloys  of  two 
constituents:  one  the  phosphide  Fe3P,  and  the  other  a  saturated  solution  of  Fe3P  in 
iron.  It  is  well  known  that  a  certain  class  of  binary  alloys  when  solidifying  give  rise 
to  the  formation  of  a  eutectic  alloy,  that  is,  of  a  mechanical  mixture  made  up  in 
definite  proportions,  of  extremely  small  plates  alternately  of  one  and  the  other  con- 
stituents, the  balance  of  the  mass  consisting  of  that  constituent  which  is  present  in 
excess  over  the  amount  required  to  form  the  eutectic  alloy.  It  is  precisely  what 
happens  in  the  case  of  iron  containing  over  1.70  per  cent  phosphorus.  The  formation 
of  eutectic  alloys  will  be  further  described  in  Lesson  XXII.  For  the  iron-phosphide 
eutectic  discovered  by  Stead  the  author  in  1901  suggested  the  name  of  "Steadite." 

Figures  2,  3,  and  4  illustrate  the  structure  of  iron  containing  respectively  1.8,  8, 
and  10.2  per  cent  phosphorus.  The  mottled  constituent  made  up  of  two  structural 
elements  in  close  juxtaposition  corresponds  in  every  case  to  the  phosphide  eutectic. 
The  background  of  Figure  2  and  the  clear  regions  of  Figure  3  are  composed  of  the 
solid  saturated  solution,  while  Figure  4  is  composed  entirely  of  the  eutectic  alloy. 


4  LESSON  XX  —  IMPURITIES  IN  CAST  IRON 

(3)  When  the  iron  contains  from  10.2  per  cent  to  15.58  per  cent  phosphorus  it  is 
composed  of  crystals  of  FesP  surrounded  by  the  eutectic  mixture  just  described,  as 
illustrated  in  Figure  5,  in  which  the  white  angular  areas  represent  Fe3P  and  the  back- 
ground the  eutectic  alloy. 

(4)  Describing  the  influence  of  carbon  on  the  structure  of  iron-phosphorus  alloys, 
Stead  wrote: 

"On  melting  saturated  solid  solutions  of  phosphide  of  iron  in  iron  with  carbon, 
the  latter  causes  a  separation  of  the  phosphide  near  to  the  point  of  solidification, 
which  appears  in  the  solid  metal  as  a  eutectic  in  irregular-shaped  areas,  if  the  carbon 
present  is  small,  and  in  envelopes,  increasing  in  thickness  with  the  amount  of  carbon 
present,  but  is  incapable  of  throwing  the  whole  of  the  phosphide  out  of  solution  even 
when  3.5  per  cent  C  is  present.  A  residuum  always  remains  in  solid  solution,  This  • 
residuum  is  smallest,  however,  when  the  carbon  is  at  a  maximum." 


Fig.  4.  —  Alloy  of  iron  and  phosphorus.  Phos- 
phorus 10.2  per  cent.  Magnified  350  diam- 
eters. (Stead.) 


P'ig.  5.  —  Alloy  of  iron  and  phosphorus.  Phos- 
phorus 11.07  per  cent.  Magnified  60  diam- 
eters. (Stead.) 


These  conclusions  are  illustrated  in  Figures  6,  7,  and  8,  showing  the  structure  of 
some  samples  of  iron  with  1.7  per  cent  phosphorus,  and  containing  respectively  0.18, 
0.71,  and  1.4  per  cent  C. 

It  will  be  obvious  from  the  above  description  of  the  behavior  of  phosphorus  that 
in  cast  iron,  because  of  the  presence  of  a  large  amount  of  carbon,  nearly  the  whole  of 
the  phosphorus  is  liberated  from  its  solution  with  iron  and  caused  to  occur  as  the 
phosphide  eutectic  or  steadite,  even  if  the  metal  contains  less  than  the  necessary 
amount  of  phosphorus  needed  to  saturate  the  iron,  namely  1.70  per  cent.  Indeed  so 
marked  is  this  action  of  carbon  that  Stead  tells  us  that  in  steel  containing  but  0.1 
per  cent  phosphorus  a  portion  of  it  is  liable  to  be  thrown  out  of  solution  in  the  pres- 
ence of  0.90  per  cent  carbon. 

To  sum  up,  the  phosphorus  in  ordinary  steels  occurs  chiefly  and  probably  alto- 
gether as  the  phosphide  FesP  dissolved  in  iron  while  in  cast  iron  it  occurs  chiefly  if  not 
entirely  as  a  eutectic  the  constituents  of  which  are  (1)  a  solid  solution  of  iron  and 
1.70  per  cent  of  phosphorus  and  (2)  the  phosphide  Fe3P.  While  Stead  writes  that  the 
whole  of  the  phosphorus  is  not  liberated  from  solution  even  in  the  presence  of  much 


LESSON  XX  —  IMPURITIES  IN  CAST  IRON 


5 


carbon,  the  amount  retained  in  solution  in  the  presence  of  some  3  per  cent  or  more 
carbon  is  apparently  very  small  and  it  may  probably  be  assumed  for  all  practical  pur- 
poses that  in  cast  iron  the  whole  of  the  phosphorus  is  present  as  a  eutectic,  for  Stead 
says  that  the  phosphide  eutectic  may  be  detected  in  cast  iron  containing  as  little  as 


Fig.  6.  —  Alloy  of  iron,  phosphorus,  and  carbon.  Fig.  7.  —  Alloy  of  iron,  phosphorus,  and  carbon. 
Phosphorus  1.74  per  cent,  carbon  0.18  per  Phosphorus  1.70  per  cent,  carbon  0.71  per 
cent.  Magnified  60  diameters.  (Stead.)  cent.  ^Magnified  250  diameters.  (Stead.) 


Fig.  8.  —  Alloy  of  iron,  phosphorus,  and  carbon. 
Phosphorus  1.70  per  cent,  carbon  1.40  per 
cent.  Magnified  250  diameters.  (Stead.) 

0.03  per  cent  phosphorus.  The  structure  of  phosphoretic  cast  iron  is  illustrated  in 
Figures  9  and  10.  The  various  constituents,  graphite,  pearlite,  free  ferrite,  and 
steadite  are  easily  distinguishable.  The  first  sample  has  a  hypo-eutectoid  matrix, 
the  .second  a  eutectoid  matrix. 

Stead  recommends  heat  tinting  as  a  suitable  treatment  for  bringing  out  the 
phosphide  eutectic  especially  in  white  cast  iron  when  there  is  danger  of  confounding 


6 


LESSON   XX  —  IMPURITIES   IN   CAST  IRON 


it  with  cementite.     The  heat  tinting  method  of  Stead  will  be  found  described  in  an 
Appendix  to  these  lessons. 

Stead  explains  as  follows  the  fact  that  a  relatively  high  proportion  of  phosphorus 
in  cast  iron  does  not  produce  extreme  brittleness. 


Fig.  9. — Cast  iron.    Magnified  100  diameters.    Graph- 
ite, ferrite,  pearlite,  and  steadite.    (Boylston.) 


Fig.    10.  —  Cast  iron.      Magnified    100   diameters. 
Graphite,  pearlite,  and  steadite.     (Boylston.) 

"The  reason  why  phosphoretic  pig  irons  are  not  more  brittle  than  they  are  is 
because  the  eutectic  separates  into  isolated  segregations,  and  does  not  form  con- 
tinuous cells  round  the  crystalline  grains.  When  the  phosphorus  does  not  exceed 
1.7  per  cent  the  metal  is  comparatively  strong,  but  an  addition  of  0.3  per  cent  reduces 
the  strength  materially,  the  reason  of  which  is  that  the  eutectic  brittle  areas  in  metal 


LESSON  XX  —  IMPURITIES  IN   CAST  IRON  7 

with  2  per  cent  phosphorus  approach  each  other  closely,  leaving  less  of  the  strong 
ground  mass  intervening." 

Phosphorus  has  no  marked  influence  upon  the  condition  in  which  carbon  occurs 
in  cast  iron  but  it  increases  the  fluidity  of  the  metal  probably  because  of  the  forma- 
tion of  a  large  quantity  of  fusible  and  fluid  phosphide  eutectic. 

Structural  Composition  of  Phosphoretic  Cast  Iron.  —  Since  the  presence  of 
10.2  per  cent  phosphorus  causes  the  production  of  100  per  cent  steadite  it  follows 
that  the  phosphorus  in  cast  iron  gives  rise  to  the  formation  of  approximately  10 
times  its  own  weight  of  steadite. 

In  calculating  the  structural  composition  of  cast  iron,  therefore,  the  amount  of 
phosphorus  present  must  be  considered  as  it  may  very  materially  lower  the  percent- 
age of  combined  carbon  needed  to  convert  its  matrix  into  pearlite.  Bearing  in  mind 
that  cast  iron  to  be  free  from  both  free  ferrite  and  free  cementite  must  contain  ferrite 
and  cementite  in  the  proportion  of  seven  to  one  the  following  relation  will  indicate 
the  needed  amount  of  combined  carbon: 

Ferrite  =  100  -  G  -  lOPh  -  15C  =  7  x  15C  =  105C 

i 
cementite 

or  100  -  G  -  lOPh  -  120C  =  0 

in  which  G,  C,  and  Ph  represent  respectively  the  percentage  of  graphite,  combined 
carbon,  and  phosphorus,  15C  representing,  of  course,  the  proportion  of  cementite  and 
lOPh  that  of  steadite. 

If  the  first  term  of  the  above  equation  is  greater  than  0  the  metal  will  contain  free 
ferrite,  i.e.  its  matrix  will  be  hypo-eutectoid;  if  it  is  smaller  than  0  it  will  contain  pure 
cementite,  i.e.  its  matrix  will  be  hyper-eutectoid. 

In  the  presence  of  3  per  cent  of  graphite  and  1  per  cent  phosphorus,  for  instance,  the 
percentage  of  combined  carbon  needed  tot  produce  a  eutectoid  matrix  will  readily  be 
obtained  in  solving  the  equation 

100  -  3  -  10  -  120C  =  0 

which  calls  for  0.75  per  cent  combined  carbon.  Less  combined  carbon  would  produce 
free  ferrite  while  more  would  cause  the  formation  of  free  cementite. 

In  calculating  the  structural  composition  of  any  phosphoretic  cast  iron  of  known 
percentage  of  phosphorus,  graphite,  and  combined  carbon,  it  should  first  be  ascertained 
therefore  whether  its  matrix  will  be  hypo-  or  hyper-eutectoid.  Let  us  assume,  for 
instance,  a  cast  iron  containing  1.50  per  cent  phosphorus,  3.25  per  cent  graphite,  and 
0.40  per  cent  combined  carbon.  Since  100  -  3.25  -  15  -  120  x  0.40  is  greater  than 
0  the  matrix  of  the  iron  will  be  hypo-eutectoid,  that  is  the  metal  will  contain  free 
ferrite.  The  following  equations  will  then  permit  the  ready  calculation  of  its  struc- 
tural composition. 

(1)  P  +  F  +  S  +  G  =  100 

(2)  P  =  120C 

(3)  S  =  lOPh 
which  give 

Pearlite  (P)  =  48.00  per  cent 

Free  ferrite  (F)  =  33.75    "       " 

Steadite  (S)  =  15.00    "      " 

Graphite  (G)  3.25    "      " 

•  100.00 


8  LESSON  XX  —  IMPURITIES  IN  CAST  IRON 

Taking  another  example,  a  cast  iron  containing  2  per  cent  graphite,  1.50  per  cent 
combined  carbon,  and  0.75  per  cent  of  phosphorus,  since  100  -  2  -  7.50  -  120  x  1.50 
is  less  than  0,  the  iron  will  contain  free  cementite.  Its  structural  composition  will 
be  obtained  by  solving  the  equations 

(1)  P  +  Cm  +  S  +  G  =  100 

(2)  P  =  f-  (100  -  15C  -  lOPh  -  G) 

(3)  S  =  lOPh 
which  give 

Pearlite  (P)  =  77.78  per  cent 

Free  cementite  (Cm)  =  12.72    "       " 
Steadite  (S)  =    7.50    "      " 

Graphite  (G)  =__2^0    "      " 

100.00 

Chemical  vs.  Structural  Composition.  —  It  will  now  be  instructive  to  compare  the 
chemical  composition  of  cast  iron  both  ultimate  and  proximate  with  its  structural 
composition.  To  that  effect  let  us  assume  a  cast  iron  of  the  following  ultimate  chemi- 
cal composition: 

Graphitic  carbon        3.00  per  cent 

Combined  carbon       0.50    "       " 

Silicon  2.00    "       " 

Phosphorus  1.50    " 

Manganese  0.40    "       " 

Sulphur  0.02    "       " 

Iron  (by  difference)  92.58    " 
100.00 

In  view  of  the  foregoing  considerations  the  following  proximate  compounds  will  be 
formed: 

(1)  0.02  per  cent  S  will  produce  about  0.05  per  cent  MnS. 

(2)  0.05  per  cent  MnS  contains  about  0.03  per  cent  Mn. 

(3)  This  leaves  0.40  —  0.03  =  0.37  per  cent  Mn  in  excess  to  combine  with  carbon. 

(4)  0.37  per  cent  Mn  will  form  0.39  per  cent  MiisC. 

(5)  0.39  per  cent  Mn  contains  about  0.02  per  cent  carbon. 

(6)  This  leaves  0.50  -  0.02  =  0.48  per  cent  carbon  to  combine  with  iron. 

(7)  0.48  per  cent  carbon  will  form  7.20  per  cent  Fe3C. 

(8)  1.50  per  cent  phosphorus  will  form  9.63  per  cent  Fe3P. 

(9)  2.00  per  cent  silicon  will  form  6  per  cent  FeSi. 

The  proximate  chemical  analysis  of  the  cast  iron  considered  will  consequently  be: 

Graphitic  carbon        3.00  per  cent 

Fe3C                             7.20  "  " 

Mn3C                           0.39  "  " 

Fe3P                           9.63  "  " 

FeSi                             6.00  "  " 

MnS                             0.05  "  " 

Iron  (by  difference)  73.73  "  " 
100.00 


LESSON  XX  —  IMPURITIES  IN  CAST  IRON  9 

Knowing  the  proximate  chemical  constituents  the  structural  composition  can  be 
readily  calculated.  The  Fe3C  and  Mn3C  form  the  cementite,  hence  the  cast  iron  con- 
tains 7.20  +  0.39  =  7.59  per  cent  cementite.  The  whole  of  this  cementite,  since  the 
iron  evidently  has  an  hypo-eutectoid  matrix,  will  combine  with  ferrite  in  the  propor- 
tion of  7  to  1  to  form  pearlite,  hence  the  cast  iron  will  contain  7.59  x  8  =  60.72  per 
cent  pearlite;  1.50  per  cent  phosphorus  means  15  per  cent  of  steadite;  the  6.00  per 
cent  of  FeSi  will  be  dissolved  in  the  ferrite  while  the  small  quantity  of  MnS  will  occur 
as  independent  minute  particles.  The  structural  composition  will  therefore  be: 

Pearlite  (P)  =  60.72  per  cent 

Free  ferrite  (F)  (by  difference)  =  21.23    "  -   -1L-. 
Steadite  (S)  =  15.00    "       " 

Graphite  (G)  =    3.00    "       " 

MnS  =    0.05    "       " 

100.00 

The  quicker  method  for  the  calculation  of  the  structural  composition  of  cast  iron 
given  in  foregoing  pages,  and  in  which  the  presence  of  manganese,  silicon,  and  sulphur 
is  ignored  would  have  given: 

Pearlite  (P)  60.00 
Free  ferrite  (F)  22.00 
Steadite  (S)  15.00 
Graphite  (G)  3.00 
100.00 

For  practical  purposes  these  values  are  identical  to  those  obtained  from  a  knowledge 
of  the  complete  analysis  of  the  iron. 

Experiments 

Samples  of  gray  cast  iron  having  respectively  hypo-  and  hyper-eutectoid  matrix  as 
well  as  a  sample  of  white  cast  iron  should  be  prepared  for  microscopical  examination. 
They  should  be  examined  both  before  and  after  etching.  In  the  unetched  condition 
the  graphite  plates  will  stand  out  sharply  on  a  brilliant,  white  background.  After 
etching  the  following  features  should  be  noted,  (1)  the  polyhedric  character  of  the 
ferrite  grains  in  cast  iron  containing  a  small  amount  of  combined  carbon,  (2)  the  in- 
creasing proportions  of  pearlite  in  cast  iron  with  hypo-eutectoid  matrix  as  the  per- 
centage of  combined  carbon  increases,  (3)  the  laminations  of  the  pearlite  areas, 
(4)  the  occurrence  of  free  cementite  in  cast  iron  containing  more  than  some  0.80  per 
cent  combined  carbon  and  (5)  the  occurrence  of  phosphide  eutectic  (steadite)  in  all 
cast  irons  containing  an  appreciable  proportion  of  phosphorus. 

The  samples  should  be  photographed  under  a  magnification  not  exceeding  100 
diameters. 

Examination 

I.     Explain  the  occurrence  of  phosphorus  in  cast  iron. 

II.  A  cast  iron  contains  2.50  per  cent  graphite,  0.75  per  cent  combined  carbon,  and 
1.25  per  cent  phosphorus.  Till  its  matrix  be  hypo-  or  hyper-eutectoid? 
Explain  your  answer. 


10  LESSON   XX  —  IMPURITIES   IN   CAST   IRON 

III.     What  will  be  (1)  the  proximate  chemical  composition  and  (2)  the  structural 
composition  of  a  cast  iron  having  the  following  chemical  ultimate  composition: 

Graphitic  carbon        2.50  per  cent 

Combined  carbon        1.00  " 

Silicon                         1.50  "  " 

Phosphorus                  1.25  "  " 

Manganese                   0.50  "  " 

Sulphur                         0.04  "  " 

Iron  (by  difference)  93.21  "  " 
100.00 


LESSON  XXI 

MALLEABLE  CAST  IRON 

Graphitizing  of  Cementite.  —  The  unstability  of  cementite  has  already  been  alluded 
to.  It  has  been  mentioned  that  the  prolonged  annealing  of  high  carbon  steel  above 
its  critical  range  was  always  likely  to  result  in  the  formation  of  some  graphitic  carbon 
through  the  dissociation  of  the  unstable  cementite  according  to  the  reaction: 

Fe3C  =  3Fe  +  C 

The  graphitic  carbon  formed  in  this  way  is  often  called,  according  to  Ledebur, 
"temper"  carbon  to  distinguish  it  from  the  graphite  formed  during  the  solidification 
of  cast  iron.  This  breaking  up  of  cementite  into  ferrite  and  graphite  takes  place  the 
more  readily  (1)  the  more  combined  carbon  in  the  metal,  (2)  the  higher  the  tempera- 
ture, (3)  the  longer  the  exposure  to  a  high  temperature,  (4)  the  more  silicon  and  the 
less  manganese  and  sulphur  present.  The  influence  of  a  large  amount  of  combined 
carbon  in  promoting  the  graphitizing  of  cementite  is  made  evident  by  the  facts  (1) 
that  iron-carbon  alloys  containing  less  than  some  0.50  per  cent  carbon  cannot  be  made 
graphitic  under  the  most  favorable  annealing  conditions  even  when  containing  much 
silicon,  (2)  that  in  steel  containing  in  the  vicinity  of  one  per  cent  carbon  the  graphi- 
tizing proceeds  very  slowly  and  remains  partial,  and  (3)  that  in  alloys  containing  2.50 
per  cent  or  more  of  combined  carbon  and  a  sufficient  amount  of  silicon  the  conversion 
of  cementite  into  iron  and  graphite  takes  place  readily  and  can  be  carried  to  comple- 
tion, combined  carbon  disappearing  altogether.  The  influence  of  temperature  and 
time  upon  the  dissociation  of  cementite  was  to  be  expected.  Evidences  will  be  pre- 
sented in  Lesson  XXIII  to  show  that  the  higher  the  temperature  at  which  cementite 
forms  the  more  readily  is  it  converted  into  iron  and  graphitic  carbon,  during  solidi- 
fication and  subsequent  cooling.  The  influence  of  silicon  in  graphiti.zing  cementite 
could  likewise  have  been  anticipated  because  of  its  well-known  power  to  cause  the 
formation  of  graphitic  carbon  during  and  below  solidification.  It  will  be  argued  in 
Lesson  XXIII  that  the  formation  of  graphite  during  solidification  always  results 
from  the  dissociation  of  cementite,  that  is,  that  cementite  (FesC)  always  forms  first 
but  being  very  unstable  readily  breaks  up  into  iron  and  graphite,  its  dissociation  being 
promoted  (1)  by  slow  cooling  and  (2)  by  the  presence  of  silicon. 

The  conversion  of  combined  carbon  into  graphitic  or  rather  temper  carbon  finds 
an  important  industrial  application  in  the  manufacture  of  so-called  malleable  cast- 
iron  castings,  also  termed  "malleable  castings,"  "malleable  cast  iron,"  and  even 
"malleable"  pronounced  "mallable." 

The  metallography  of  these  castings  should  now  be  considered. 

Malleable  Cast-Iron  Castings.  —  The  adjective  "malleable"  always  used  in  de- 
scribing these  castings  is  misleading  for  it  suggests  a  malleability,  and  other  proper- 
ties akin  to  those  of  steels,  which  malleable  castings  are  far  from  possessing.  By 
malleability  the  metallurgist  always  understands  that  property  which  makes  it  pos- 

1 


2  LESSON  XXI  —  MALLEABLE  CAST  IRON 

sible  to  convert  a  metallic  mass  into  commercial  shapes  by  rolling,  hammering,  etc., 
and  such  degree  of  malleability  is  not  present  in  malleable  cast  iron.  It  is  why  in  the 
author's  opinion  this  product  should  continue  to  be  classified  as  cast  iron  in  spite  of 
the  arguments  recently  presented  to  classify  it  as  steel  on  the  ground  that  it  is  more 
malleable  than  ordinary  cast  iron.  The  manufacture  of  malleable  cast-iron  castings 
consists  in  subjecting  to  a  long  annealing  treatment  cast-iron  castings  of  suitable  com- 
position whereby  some  of  the  carbon  may  be  expelled  by  oxidation  while  most  if  not 
the  whole  of  the  remaining  is  converted  into  graphite  (temper  carbon).  The  various 
factors  influencing  this  operation  will  be  briefly  described. 

Original  Castings.  —  The  castings  to  be  subjected  to  the  malleablizing  treatment 
are  often  called  "hard"  castings  because  they  are  invariably  made  of  white  cast  iron 
and  are  therefore  very  hard  and  brittle.  The  reason  why  the  original  castings  must 
be  made  of  white  cast  iron  will  be  obvious  if  it  be  considered  that  the  malleability  to 
be  imparted  to  these  castings  is  to  result  chiefly,  and  in  some  cases  altogether,  from  a 
conversion  of  combined  carbon  into  temper  carbon  as  explained  above.  Any  graphite 
particle  existing  in  the  casting  will  be  unaffected  by  the  annealing  treatment  and  will 
be  a  source  of  weakness  in  the  finished  casting.  Clearly,  therefore,  the  hard  castings 
should  be  free  from  graphitic  carbon.  As  to  the  amount  of  carbon  that  should  be 
present,  theoretical  considerations  lead  us  to  conclude  that  the  less  carbon  the  more 
ductile  should  be  the  casting  after  thorough  malleablizing,  for,  after  all,  the  particles  of 
temper  carbon  while  much  less  injurious  than  the  hard  brittle  cementite  from  which 
they  are  derived  are  nevertheless  a  source  of  weakness  chiefly  because  of  their  breaking 
up  the  continuity  of  the  metallic  mass.  Again  a  smaller  proportion  of  carbon  neces- 
sarily means  a  shorter  annealing  operation.  The  desirability  of  low  carbon  content  in 
cast-iron  castings  to  be  malleablized  is  universally  recognized  and  it  is  partly  why  a 
large  proportion  of  these  castings  are  made  in  the  "air"  furnace,  as  by  its  use  the  per- 
centage of  carbon  can  be  lowered.  On  the  other  hand,  as  already  mentioned,  if  the 
proportion  of  combined  carbon  be  small  the  graphitizing  takes  place  with  greater  diffi- 
culty. These  considerations  point  to  the  existence  of  a  lower  as  well  as  of  an  upper 
limit  for  the  carbon  content  of  hard  castings.  In  the  majority  of  cases  the  percentage 
of  carbon  varies  between  2.50  and  3.00  per  cent. 

The  beneficial  action  of  silicon  has  been  referred  to;  it  greatly  promotes  the  graphi- 
tizing of  the  cementite.  It  might  seem  then  as  if  the  more  silicon  in  the  hard  casting 
the  better  at  least  up  to  some  2  or  3  per  cent.  Castings  of  white  cast  iron  cannot  be 
made,  however,  in  the  presence  of  much  silicon  since  this  element  would  cause  the 
formation  of  graphitic  carbon  on  solidification,  even  during  quick  cooling,  and  the 
casting  would  not  be  white.  And  since  large  castings  will  solidify  more  slowly  than 
smaller  ones  it  is  evident  that  in  large  castings  especially  but  a  relatively  small  amount 
of  silicon  can  be  allowed.  It  is  probably  true  that  as  much  silicon  should  be  intro- 
duced in  the  cast  iron  as  will  permit  the  making  of  white  castings  and  this  proportion 
of  silicon  will  necessarily  decrease  as  the  size  of  the  casting  increases.  In  practise  it 
is  found  to  vary  between  0.30  per  cent  in  large  castings  (one  inch  thick  or  more)  and 
1.25  per  cent  in  very  small  castings  for  like  solidification  conditions  and  subsequent 
cooling.  In  the  majority  of  cases  the  silicon  content  varies  between  0.50  and  1  per 
cent. 

In  Moldenke's,  opinion  the  manganese  should  not  exceed  0.60  per  cent  as  a  larger 
amount  is  liable  to  cause  trouble  on  annealing  and  difficulty  in  machining.  Phos- 
phorus according  to  the  same  authority  should  not  exceed  0.225  per  cent  while  sulphur 
should  not  exceed  0.07  and  preferably  not  0.05  per  cent.  These  figures  represent 


LESSON   XXI  —  MALLEABLE   CAST   IRON  3 

American  practise  only,  in  Europe  cast-iron  castings  being  malleablized  containing 
considerably  more  sulphur  and  phosphorus. 

The  structure  of  a  hard  casting  of  suitable  composition  for  the  malleablizing  process 
is  shown  in  Figure  1.  It  will  be  seen  to  be  characteristic  of  the  structure  of  white  cast 
iron. 

Annealing  Operation.  —  The  hard  castings  are  placed  in  an  annealing  box  and 
firmly  packed  in  a  suitable  material.  The  boxes  are  then  placed  in  the  annealing 
furnaces  and  with  their  contents  exposed  to  a  desirable  temperature  for  a  suitable 
length  of  time.  These  various  steps  should  be  briefly  considered. 

Packing  Materials.  —  The  packing  material  affords  a  support  for  the  castings 
while  it  may  or  may  not  play  an  important  part  in  the  process  Tfself  according  to  its 
being  (1)  oxidizing  or  (2)  non-oxidizing.  In  his  pioneer  work,  in  1722,  Reaumur 
surrounded  white  cast  iron  with  crushed  iron  oxide,  the  oxygen  of  which  removed  a 


Fig.  1.  —  Hard  casting  (white  cast  iron).     Magnified 
67  diameters.     (Boylston.) 

large  proportion  of  the  carbon  during  the  subsequent  long  annealing  treatment  at  a 
high  temperature.  Given  a  sufficiently  high  temperature  and  sufficiently  long  expo- 
sure nearly  the  whole  of  the  carbon  could  in  this  way  be  eliminated  from  small  castings, 
a  small  amount  of  it  only  remaining  as  temper  carbon.  It  will  be  evident  that  in  the 
operation  conducted  in  this  way  the  oxidizing  action  of  the  packing  is  of  more  im- 
portance in  producing  malleability  than  the  graphitizing  of  the  cementite.  The 
elimination  of  the  carbon  takes  place  through  the  well-known  reaction  between  the 
iron  oxide  (from  the  packing)  and  the  carbon  in  the  steel, 

Fe3C  +  3O  =  3Fe  +  3CO 

the  cementite  in  the  center  migrating  towards  the  outside  and  being  in  turn  acted 
upon  by  the  oxygen  of  the  packing  material. 

It  will  be  evident  that  this  elimination  must  necessarily  proceed  from  the  outside 
to  the  center,  so  that  if  one  should  interrupt  the  operation  after  a  relatively  short 
time  he  would  find  an  increasing  proportion  of  carbon  from  shell  to  center.  The  use 


4  LESSON   XXI  —  MALLEABLE   CAST   IRON 

of  an  oxidizing  and  therefore  chemically  active  packing  material  was  for  a  long  time 
considered  essential  in  the  malleablizing  process.  Later  investigations,  however, 
showed  that  white  cast  iron  could  be  made  malleable  solely  through  the  conversion 
of  combined  into  graphitic  carbon  with  very  little  if  any  removal  of  the  carbon  and 
that  an  oxidizing  packing  was  not  therefore  necessary.  Inert  packing  such  as  sand 
and  clay  may  be  used  and  satisfactory  malleable  castings  produced.  It  is  customary, 
however,  in  American  practise,  to  use  oxidizing  packings  but  to  depend  chiefly  on  the 
graphitizing  of  cementite  brought  about  by  annealing  for  the  desired  malleability  and 
strength.  Although  realizing  that  an  oxidizing  substance  as  a  packing  material  is 
not  indispensable  it  is  still  preferred  because  of  the  apparent  greater  strength  its  use 
confers  to  the  castings.  The  substances  most  used  are  powdered  iron  oxides  in  the 
shape  of  iron  ore,  mill  scale,  "bull-dog,"  etc.,  and,  according  to  Moldenke,  the  crushed 
flakes  detached  from  the  annealing  pots  themselves. 

Annealing  for  Malleablizing.  —  The  malleability  imparted  to  white  cast  iron  by 
annealing  results  fro*n,  as  explained  in  the  foregoing  pages,  (1)  the  hard  brittle  cemen- 
tite being  converted  into  minute  rounded  particles  of  soft  graphitic  (temper)  carbon 
and  (2)  the  burning  of  some  carbon  by  the  oxygen  of  the  packing  materials.  If  we 
depend  chiefly  upon  the  burning  of  the  carbon  for  the  desired  malleability,  so-called 
"white  heart"  castings  are  produced,  while  if  the  graphitizing  of  cementite  is  mainly 
sought,  so-called  "black  heart"  castings  are  obtained.  The  temperature  and  length 
of  the  annealing  operation  vary  according  to  the  kind  of  castings  wanted. 

The  annealing  operation  is  always  conducted  above  the  critical  point  Ac3.2.i  of  the 
white  cast  iron  and  of  course  below  its  solidification,  that  is  when  the  metal  exists  as 
an  aggregate  of  cementite  and  solid  solution  (saturated  austenite)  as  explained  in 
Lesson  XXIII.  It  is  not  known  whether  the  free  or  the  dissolved  cementite  is  graph- 
itized  first  or  whether  the  dissociation  of  both  kinds  proceeds  simultaneously.  Noting, 
however,  that  the  graphitizing  takes  place  more  readily  in  the  presence  of  a  consider- 
able quantity  of  combined  carbon  and,  therefore,  of  free  cementite,  whereas  in  the 
presence  of  but  little  free  cementite  (cast  iron  with  less  than  2  per  cent  carbon)  it  is  at 
best  very  slow  and  remains  partial,  it  seems  logical  to  infer  that  it  is  the  free  cementite 
which  is  first  decomposed,  indeed  that  its  presence  is  necessary  to  cause  the  graphiti- 
zing of  the  dissolved  cementite. 

Annealing  for  "White  Heart  "  Castings.  —  In  the  making  of  white  heart  castings  a 
very  large  proportion  of  the  carbon  is  removed  by  oxidation  through  the  use  (1)  of  an 
oxidizing  packing,  (2)  of  a  high  annealing  temperature,  and  (3)  of  a  long  annealing. 
The  original  malleable  castings  made  by  Reaumur  were  of  this  type  and  his  method 
is  still  the  prevailing  one  in  Europe.  The  castings  are  annealed  for  four  or  five  days 
at  a  temperature  of  800  to  900  deg.  C.  By  removing  some  castings  from  the  annealing 
box  from  time  to  time  and  examining  their  structure  the  progress  of  the  operation 
may  be  readily  observed.  The  following  transformations  are  noted  as  the  operation 
progresses:  (1)  a  narrow  white  rim  of  decarburized  metal  caused  by  the  burning  of 
the  carbon  from  the  outside  of  the  casting  and  a  dark  center  caused  by  the  graph- 
itizing of  some  of  the  cementite,  (2)  a  broader  white  rim  and  a  smaller  and  darker 
core  due  to  the  burning  of  more  carbon  and  to  the  formation  in  the  center  of  the  casting 
of  more  graphitic  carbon,  the  graphitizing  of  the  cementite  being  now  indeed  possibly 
complete,  (3)  the  fracture  of  the  metal  is  now  white  and  steely  to  the  very  center 
showing  that  most  of  the  carbon  has  been  removed  by  oxidation.  These  conclusions 
are  fully  confirmed  by  the  microscopical  examination  of  the  structure  of  castings 
subjected  to  this  annealing  treatment  for  various  lengths  of  time.  As  this  oxidation 


LESSON   XXI  —  MALLEABLE   CAST   IRON  5 

of  the  carbon  is  necessarily  a  very  slow  process,  white  heart  castings  of  small  size  only 
are  made,  generally  not  over  Yi  inch  thick.  For  malleablizing  larger  castings  the 
"black  heart"  process  soon  to  be  described  is  decidedly  superior. 

White  heart  castings  have  a  rather  coarse  fracture  and  structure  because  of  the 
high  and  prolonged  heating  to  which  they  were  exposed.  Since  they  resemble  low 
carbon  steels  in  composition  and  structure  it  would  seem  as  if  their  properties  could 
be  very  materially  improved  by  suitable  heat  treatment  as,  for  instance,  by  annealing 
followed  by  cooling  in  oil  or  in  air  according  to  their  carbon  content.  Moldenke 
states  that  the  European  white  heart  malleable  castings  bend  excellently  and  are  very 
good,  though  slightly  weaker  than  black  heart  castings. 

Annealing  for  "Black  Heart"  Castings.  —  For  the  production  of  black  heart  cast- 
ings, since  the  oxidation  of  carbon  is  of  secondary  importance,  the  annealing  tem- 
perature needs  not  be  so  high  nor  indeed  is  it  necessary  to  use  an  oxidizing  packing. 
As  already  mentioned,  oxidizing  packings  are  generally  used,  however,  because  of  the 
apparent  greater  strength  they  impart  to  the  castings.  A  certain  amount  of  carbon, 
therefore,  is  burnt  at  the  surface  of  the  casting  causing  a  narrow  white  rim  while  the 
core  is  very  dark,  hence  the  name  of  black  heart  given  to  these  castings.  This  dark- 
ness of  the  core  is  due  as  we  now  understand  it  to  the  transformation  of  cement 
carbon  into  temper  carbon. 

The  annealing  temperature  for  the  production  of  black  heart  castings  is  generally 
between  750  and  800  deg.  C.  in  the  case  of  air  furnace  iron,  that  is,  but  slightly  above 
the  critical  range  of  the  metal  and  the  length  of  time  at  the  full  converting  tempera- 
ture between  2]/2  and  3  days.  Moldenke  states  that  cupola  iron  castings  should  be 
annealed  at  temperatures  some  65  to  120  deg.  higher  and  writes: 

"Just  why  there  should  be  a  difference  in  the  temperature  required  for  castings 
of  the  same  composition  when  made  in  the  cupola  or  in  the  air  furnace,  is  one  of  the 
unsolved  problems.  It  may  be  chemical  in  that  the  degree  of  oxidation  has  its  effect 
on  the  opening  up  of  the  structure  under  the  influence  of  heat.  It  may,  on  the  other 
hand,  be  a  matter  of  molecular  physics,  and  depend  on  the  constitution  and  structure 
of  the  castings  as  made,  either  in  the  contact  of  fuel  with  iron,  or  not.  Possibly  it 
may  be  a  combination  of  both  the  chemical  and  the  physical.  Yet  the  problem  still 
remains  to  be  solved." 

In  completely  malleablized  black  heart  castings  practically  the  whole  of  the  car- 
bon should  be  present  in  the  graphitic  condition,  the  castings  consisting  then  of  a 
narrow  case  of  decarburized  iron  and  of  a  core  made  up  of  ferrite  and  particles  of 
temper  carbon,  as  shown  in  Figures  2  and  3.  If  the  malleablizing  is  but  partial 
because  of  too  low  a  temperature,  too  short  a  treatment,  too  little  silicon,  or  for  some 
other  reason,  considerable  dissolved  carbon  may  remain  which  in  cooling  through  the 
critical  range  will  give  rise  to  the  formation  of  pearlite.  The  structure  of  such  par- 
tially malleablized  cast  iron  is  illustrated  in  Figures  4  to  6.  It  will  be  seen  to  con- 
sist of  graphitic  carbon,  pearlite,  and  ferrite.  The  mechanism  of  the  formation  of 
such  structures  is  obvious.  At  the  end  of  the  annealing  treatment  the  metal  con- 
sisted of  a  mass  of  austenite  in  which  were  embedded  a  number  of  particles  of 
temper  carbon;  on  slow  cooling  through  the  range  the  hypo-eutectoid  austenite 
rejected  some  free  ferrite  until,  its  composition  reaching  the  eutectoid  carbon  ratio, 
it  was  converted  intb  pearlite.  It  will  be  noted  that  the  pearlitic  areas  which  should 
indicate  the  location  of  the  residual  austenite  are  situated  away  from  the  temper 
carbon  particles,  the  latter  being  surrounded  by  ferrite. 

The  annealing  of  white  cast  iron  may  be  so  incomplete  as  to  retain  so  much  dis- 


6  LESSON  XXI  —  MALLEABLE  CAST  IRON 

solved  carbon  that  in  slow  cooling  free  cementite  as  well  as  pearlite  will  be  formed. 
This  is  well  shown  in  Figure  7.     In  this  case  the  austenito  which  existed  above  the 


Fig.  2. — Black  heart  casting.     Magnified  100  diameters.     Not  etched 
(F.  C.  Langenbcrg  in  the  author's  laboratory.) 


g%^prA,<, 


*5k\*k/*  k-v  j 
•       .^\'  V   -J 
"  '-W-  -rv. 

**/*  -IvM^ 


Fig.  3.  —  Same  metal  as  in  Fig.  2.     Magnified  100  diameters.     Etched 
(F.  C.  Langenberg  in  the  author's  laboratory.) 

range  at  the  end  of  the  annealing  operation  was  hyper-eutectoid,  that  is,  it  contained 
more  than  0.85  per  cent,  or  thereabout,  of  dissolved  carbon  and  on  cooling  through 


LESSON   XXI  —  MALLEABLE   CAST   IRON 


the  range,  therefore,  liberated  some  free  cementite.  It  is  evident  that  partially 
malleablized  cast  iron,  that  is,  cast  iron  still  containing  considerable  combined  carbon, 
cannot  be  as  malleable  as  malleablized  cast  iron  free  from  combined  carbon  since  its 
metallic  matrix  is  necessarily  less  malleable. 


Fig.  4.  —  White  cast  iron  partially  malleablized.  Magnified  100  diam- 
eters. Particles  of  temper  carbon  surrounded  by  white  ferrite.  The 
dark  background  is  pearlite.  (F.  C.  Langenberg  in  the  author's 
laboratory.) 


Fig.  5.  —  White  cast  iron  partially  mal- 
leablized. Magnified  50  diameters. 
(Wiist.) 


Fig.  6.  —  Same  metal  as  in  Fig.  5.     Magni- 
fied 500  diameters.     (Wiist.) 


Gray  Cast  Iron  vs.  Malleable  Cast  Iron.  —  From  the  foregoing  description  of  the 
nature  and  manufacture  of  malleable  cast-iron  castings  it  is  evident  that  gray  cast 
iron  and  malleable  cast  iron  may  have  exactly  the  same  chemical  composition,  although, 
of  course,  the  fc.rmer  will  generally  contain  more  silicon  and  total  carbon.  In  spite 
of  identical  or  nearly  identical  composition,  however,  these  two  metals  differ  enor- 
mously in  physical  properties,  gray  cast  iron  being  weak  and  brittle,  malleable  cast  iron 


8  LESSON   XXI  — MALLEABLE   CAST   IRON 

much  stronger,  and  endowed  with  remarkable  shock-resisting  qualities.  To  account 
for  this  we  must  look  into  the  microstructure  of  both  metals  when  it  will  be  observed 
that  in  gray  cast  iron  the  graphite  occurs  in  large,  generally  curved,  flakes  and  plates 
whereas  in  malleable  cast  iron  it  is  present  in  small  rounded  particles,  and  it  may  well 
be  conceived  that  the  mode  of  occurrence  of  graphite  in  gray  iron  breaks  up  the 
continuity  of  the  metallic  matrix  much  more  effectively,  therefore,  weakening  it  and 


Fig.  7.  —  White  cast  iron  slightly  malleablized.  Mag- 
nified 100  diameters.  (H.  F.  Miller  in  the  author's 
laboratory.) 

destroying  its  ductility.  Were  it  possible  during  the  solidification  of  cast  iron  to 
cause  the  graphite  to  occur  as  it  does  in  malleable  cast  iron,  there  is  no  reason  to  doubt 
but  that  it  would  be  as  strong  and  as  ductile. 

Experiments 

Samples  of  fully  and  of  partially  malleablized  cast  iron  should  be  prepared  for 
microscopical  examination  as  well  as  a  sample  of  white  cast  iron  suitable  for  the  nialle- 
ablizing  process.  They  should  be  examined  both  before  and  after  etching  and  their 
structures  compared  with  the  illustrations  of  this  lesson.  The  following  features 
should  be  noted:  (1)  the  decarburized  shells  surrounding,  the  graphitic  cores  of  the 
malleablized  castings,  (2)  the  polyhedric  structure  of  the  ferrite  of  fully  malleablized 
cast  iron,  (3)  the  ferrite  areas  surrounding  the  temper  carbon  particles  of  partially 
malleablized  samples  having  an  hypo-eutectoid  matrix,  and  (4)  the  occurrence  of  free 
cementite  in  very  incompletely  malleablized  castings. 

Examination 

I.     Explain  the  graphitizing  of  cementite. 
II.     Describe  the  structure  of  (1)  fully  malleablized  black  heart  castings  and  (2)  of 

partially  malleablized  black  heart  castings. 

III.  Explain  the  apparent  reason  for  the  malleability  of  annealed  white  cast  iron 
and  the  lack  of  malleability  of  gray  cast  iron  containing  the  same  proportion 
of  graphitic  carbon. 


LESSON   XXII 

CONSTITUTION  OF  METALLIC  ALLOYS 

For  many  years  vague  and  conflicting  opinions  were  entertained  in  regard  to  the 
nature  of  metallic  alloys.  It  was  not  known  whether  these  intimate  associations  of 
two  or  more  metals  were  merely  mechanical  mixtures  or  chemical  compounds  while 
the  existence  of  solid  solutions  was  unsuspected.  The  application  to  the  study  of 
metallic  alloys  of  the  determination  of  the  solubility  curves  which  had  proved  so 
fruitful  in  investigating  the  mechanism  of  the  solidification  of  ordinary  (liquid)  solu- 
tions and  of  mixtures  of  melted  salts,  soon  followed  by  the  microscopical  examination 
of  their  structure  have  at  last  revealed  their  true  constitution.  We  know  now  that 
metallic  alloys  may  be  considered  as  solution  of  high  freezing  (solidification)  point 
and,  therefore,  solid  at  ordinary  temperature,  whereas  the  old  conception  of  solution 
applied  only  to  substances  liquid  at  that  temperature.  It  is  evident,  however,  that 
the  location  of  the  freezing-point  of  a  substance  in  the  temperature  scale  can  have 
no  bearing  whatever  upon  its  constitution,  that  is,  upon  the  mode  of  occurrence 
of  its  constituents  and  the  nature  of  the  bond  uniting  them. 

In  these  lessons  the  constitution  of  metallic  alloys  will  be  considered  only  so  far  as 
necessary  to  understand  the  equilibrium  diagram  described  in  Lesson  XXIII  in  which 
steel  and  cast  iron  are  considered  as  alloys  of  iron  and  carbon,  i.e.  as  solutions  of  these 
elements,  liquid  at  a  very  high  temperature,  frozen  at  ordinary  temperature. 

Since  moreover  steel  and  cast  iron  are  considered  as  pure  iron-carbon  alloys  it  will 
suffice  for  our  purpose  to  deal  only  with  alloys  of  two  metals,  that  is,  with  binary 
alloys. 

The  constitution  of  alloys  is  revealed  chiefly  (1)  through  the  mechanism  of  their 
solidification  as  disclosed  by  their  "fusibility"  curves,  and  (2)  through  the  microscopi- 
cal examination  of  their  structure  after  solidification. 

Solidification  of  Pure  Metals.  —  Let  us  first  consider  the  solidification  of  a  pure 
metal  by  observing  its  rate  of  cooling  from  the  molten  to  the  solid  condition.  This 
involves  the  use  of  a  pyrometer,  preferably  a  thermo-electric  (Le  Chatelier)  instru- 
ment, the  hot  junction  of  which,  suitably  protected,  is  embedded  in  the  cooling  metal. 
By  recording  the  successive  intervals  of  time  in  seconds  required  for  each  successive 
cooling  through  ranges  of  temperature  say  of  10  deg.  C.,  and  plotting  time  against 
temperature  a  cooling  curve  of  the  type  shown  in  Figure  1  is  obtained.1  The  teach- 
ings of  such  curves  are  obvious.  Starting  with  the  molten  metal  at  A,  its  temperature 
being  T,  its  cooling  from  A  to  B,  while  its  temperature  is  falling  from  T  to  Ts  is  uni- 
formly retarded.  This  results  in  the  smooth,  nearly  straight  portion  AB  of  the  curve. 
The  cooling  of  a  molten  metal  above  its  solidification  point  is  in  this  respect  similar 
to  the  cooling  of  any  substance  free  from  thermal  critical  points;  the  cooling  curves 

1  Self-recording  pyrometers  may  also  be  used. 


2  LESSON  XXII  —  CONSTITUTION  OF  METALLIC  ALLOYS 

obtained  in  such  cases  are  always  smooth  curves  indicating  a  uniform  increase  in 
time  as  the  temperature  of  the  substance  falls.  The  curve  of  Figure  1  indicates  the 
occurrence  of  a  sharp  critical  point  at  the  temperature  Ts  corresponding  to  the  hori- 
zontal portion  BC  of  the  curve.  It  is  evident  that  on  reaching  Ts  the  temperature 
of  the  metal  suddenly  ceased  to  fall  and  remained  stationary  during  an  interval  of 
time  represented  by  tt'.  The  metal  then  resumed  a  normal  rate  of  cooling  which  was 
continued  to  atmospheric  temperature  as  indicated  by  the  smooth  portion  CD  which, 
were  it  not  for  the  jog  BC,  would  be  a  continuation  cf  AB.  We  naturally  connect  this 
sadden  appearance  of  a  critical  point  in  the  cooling  curve  with  the  solidification  of  the 


T 


<D 

8- 


c 


D 


77 


Fig.  1.  —  Typical  cooling  curve  of  pure  metal. 


metal.  It  clearly  indicates  (1)  that  the  metal  begins  to  solidify  at  the  temperature 
Ts,  (2)  that  while  it  is  solidifying  its  temperature  remains  constant,  (3)  that  the 
solidification  lasts  t'  - 1  seconds.  It  is  evident  that  during  its  solidification  the  metal 
was  exposed  to  the  same  cooling  influences  as  those  prevailing  above  and  below  it,  and 
since  its  temperature  nevertheless  remains  constant  it  must  be  that  heat  is  here 
liberated  in  amount  exactly  sufficient  to  make  up  for  the  heat  lost  by  radiation  and 
conductivity.  Any  attempt  at  increasing  the  rate  of  cooling  during  solidification  would 
result  in  hastening  solidification  and  not  in  lowering  the  temperature  of  the  metal. 
The  heat  evolved  during  the  solidification  of  a  substance  is  known  as  its  "latent  heat 
of  solidification."  In  Figure  1,  AB  then  represents  the  cooling  of  the  liquid  metal, 
BC  its  solidification,  and  CD  the  cooling  of  the  solidified  metal.  On  heating  a  pure 
metal  from  below  to  above  its  melting-point,  a  similar  curve  is  obtained  indicating 
(1)  that  the  melting-point  coincides  with  the  solidification  point,  at  least  under  normal 


LESSON   XXII  —  CONSTITUTION   OF   METALLIC   ALLOYS 

conditions,  and  (2)  that  the  melting  takes  place  at  a  constant  temperature,  being, 
therefore,  accompanied  by  an  absorption  of  heat. 

In  Figure  2  the  cooling  curves  of  several  pure  metals  are  shown.  They  differ  only 
in  regard  to  the  location  of  the  horizontal  portions  indicating  the  temperatures  of 
solidification. 

The  structure  of  pure  metals  has  been  described  in  Lesson  I,  when  they  were  shown 
to  be  made  up  of  polyhedral  crystalline  grains,  each  grain  consisting  of  true  crystals 
of  uniform  orientation.  As  an  instance  of  the  structure  of  pure  metals,  the  struc- 
ture of  pure  lead  is  shown  in  Figure  3. 


2.OOO 


7~  /  nn  e 

Fig.  2.  —  Cooling  curves  of  various  pure  metals. 


Solidification  of  Binary  Alloys  the  Constituents  of  which  Form  Solid  Solutions.  — 
The  cooling  or  solidification  curves  of  alloys  of  two  or  more  metals  may  be  constructed 
exactly  like  the  cooling  curve  of  a  pure  metal,  namely,  by  observing  the  rate  of  cooling 
as  the  temperature  is  lowered  from  above  the  melting-point  to  atmospheric  temperature 
and  plotting  the  intervals  of  time  against  the  corresponding  temperature  falls.  A 
number  of  binary  alloys  are  then  found  to  yield  cooling  curves  of  the  type  shown  in 
Figure  4.  From  A  to  B,  that  is,  as  the  alloy  cools  from  T  to  Tb,  the  curve  is  smooth 
and,  therefore,  indicative  of  normal  cooling.  At  B  there  is  a  sudden  change  of  direction 
and  from  B  to  C,  that  is,  from  the  temperature  Tb  to  the  temperature  Tc,  the  cooling  of 
the  alloy  is  evidently  abnormally  slow.  From  C  to  D,  that  is,  from  the  temperature 
Tc  to  atmospheric  temperature,  the  cooling  is  again  normal.  Since  the  portion  BC 
of  the  curve  clearly  indicates  spontaneous  evolutions  of  heat  causing  a  marked  retarda- 
tion in  the  cooling  of  the  alloy  and  lasting  t'~t  seconds,  we  naturally  infer  that  it  cor- 
responds to  its  solidification.  It  follows  from  the  appearance  of  the  cooling  curve 


4  LESSON  XXII  —  CONSTITUTION  OF  METALLIC  ALLOYS 

that  alloys  yielding  such  curves  do  not,  like  pure  metals,  solidify  at  a  constant  tempera- 
ture but  that  their  solidification,  on  the  contrary,  lasts  t'-t  seconds  while  their  tempera- 
ture falls  from  Tb  to  Tc.  Summing  up,  AB  indicates  the  cooling  of  the  molten  alloy, 
B  the  beginning,  and  C  the  end  of  its  solidification,  tt'  the  time  required  for  its  solidifi- 
cation, Tb  Tc  the  fall  of  temperature  during  solidification,  and  CD  the  cooling  of  the 
solidified  alloy.  Above  the  point  B,  therefore,  the  alloy  is  entirely  liquid,  below  C  it 
is  entirely  solid,  while  between  B  and  C  it  is  partly  liquid  and  partly  solid.  The  point  B 
is  accordingly  called  the  liquidus  point  and  C  the  solidus  point. 

Binary  alloys  whose  cooling  curves  are  of  the  type  shown  in  Figure  4  are  known 
to  be  solid  solutions.  In  these  alloys  the  component  metals  which  are  completely 
merged  in  the  liquid  condition  remain  likewise  so  completely  merged  after  solidifica- 
tion that  their  separate  existence  cannot  be  detected  by  microscopical  examination  or 


Fig.  3.  —  Pure  lead.     Magnified  20  diameters.     (F.  C. 
Langenberg  in  the  author's  laboratory.) 

other  physical  means.  They  formed,  on  solidifying,  homogeneous  crystals  containing 
both  metals  in  indefinite  proportions.  These  crystals  are  sometimes  called  "mixed 
crystals"  and  substances  yielding  them  " isomorphous "  mixtures  by  which  it  is  meant 
that  only  isomorphous  substances '  can  yield  mixed  crystals  or  in  other  words  can  form 
solid  solutions.  The  expression  "solid  solution"  is  much  preferable  and  is  now  quite 
universally  used. 

The  mechanism  of  the  formation  of  solid  solutions  of  two  metals  should  be  ex- 
amined more  closely.  Let  us  assume  that  a  certain  proportion  of  the  metal  M  of 
relatively  low  melting-point  is  alloyed  with,  or  dissolved  in,  the  metal  M'  of  higher 
melting-point.  The  metal  M  may  be  considered  as  the  solute  and  M'  as  the  solvent. 
It  is  believed  that  when  solidification  begins  homogeneous  crystals  of  M  and  M'  are 
formed  but  that  they  contain  a  smaller  proportion  of  the  fusible  metal  M  than  the 

1  Isomorphous  substances  are  those  that  are  capable  of  crystallizing  in  the  same  crystallographic 
forms. 


LESSON  XXII  —  CONSTITUTION  OF  METALLIC  ALLOYS  5 

liquid  bath,  which  is  thereby  enriched  in  M .'  On  further  cooling  these  crystals  grow 
but  the  crystalline  matter  now  deposited  contains  more  of  the  metal  M  than  the  crystals 
first  formed,  although  still  less  than  the  molten  bath  which  is  further  enriched  in  M 
and  so  on,  the  crystals  growing  through  successive  additions  of  crystalline  matter  con- 
taining increasing  proportions  of  the  dissolved  and  relatively  fusible  metal  M,  and 
approaching,  therefore,  although  not  reaching,  the  composition  of  the  molten  metal 
until  finally  the  last  drop  solidifies.  Meanwhile,  as  the  temperature  is  lowered  through 
and  below  the  solidification  range,  diffusion  takes  place  within  the  crystals  so  that 
finally  they  become  chemically  homogeneous  provided  time  be  given  (through  slow 


Fig.  4.  —  Typical  cooling  curve  of  binary  alloy  whose  component  metals  form  a 

solid  solution. 

cooling)  for  complete  diffusion.     Like  pure  metals,  alloys  whose  component  metals 
form  solid  solutions,  are  composed  of  polyhedral  crystalline  grains  (see  Fig.  5). 

Fusibility  Curves  of  Binary  Alloys  whose  Component  Metals  are  Completely 
Soluble  in  each  Other  when  Solid.  —  So-called  "fusibility  curves"  or  "equilibrium 
diagrams"  are  obtained  from  any  series  of  alloys  by  constructing  the  solidification 
curves,  as  explained  above,  of  a  number  of  alloys  of  that  series  and  plotting  on  a  single 

1  It  is  because  the  bath  becomes  richer  in  the  more  fusible  metal  M  that  its  melting-point  is 
lowered,  resulting  in  its  solidification  covering  a  considerable  and  falling  range  of  temperature.  If 
the  crystals  first  formed  had  the  same  composition  as  the  bath,  solidification  would  take  place  at  a 
constant  temperature.  Witness  the  solidification  of  pure  metals,  of  eutectic  alloys,  and  of  chemical 
compounds:  the  solidifying  metal  having  the  same  composition  as  the  liquid  from  which  it  forms, 
solidification  takes  place  at  a  constant  temperature. 


6  LESSON  XXII  —  CONSTITUTION  OF  METALLIC  ALLOYS 

diagram  the  evolutions  of  heat  observed.  This  has  been  done  in  Figure  6  by  uniting 
the  BB'B"  .  .  .  and  the  CC'C"  .  .  .  points  denoting  respectively  the  beginning  and  the 
end  of  the  solidification  of  a  number  of  alloys  A  A' A"  .  .  .  Leaving  out  the  independent 
cooling  curves  used  for  the  construction  of  the  fusibility  curve,  the  diagram  shown  in 
Figure  7  is  obtained,  the  co-ordinates  being  now  temperature  and  composition.  The 
figure  is  typical  of  the  fusibility  curves  of  binary  alloys  whose  component  metals  are 
entirely  soluble  in  each  other  when  solid.  They  are  composed  of  two  branches,  one 
concave  the  other  convex,  uniting  the  melting-points  of  the  constituent  metals. 
MLM'  is  known  as  the  liquidus  because  any  alloy  of  the  series  above  that  line  is 
entirely  liquid,  while  MSM'  is  called  the  solidus  because  any  alloy  below  it  is  entirely 
solid.  Within  the  area  MLM'SM  the  alloys  are  partly  liquid  and  partly  solid.  The 
solidification  of  these  alloys  should  be  further  described  with  the  help  of  this  diagram. 
Let  us  assume  an  alloy  the  composition  of  which  is  represented  by  the  point  P  in  the 


Fig.  5.  —  Copper-zinc  alloy.  Copper  50  per 
cent.  Magnified  200  diameters.  Homogene- 
ous solid  solution.  (Guillet.) 

diagram  and  containing,  therefore,  75  per  cent  of  the  low  melting-point  metal  M  and 
25  per  cent  of  the  less  fusible  metal  M'.  An  alloy  of  that  composition  at  the  tempera- 
ture T  is  entirely  liquid  since  its  condition  is  represented  by  a  point  situated  above  the 
liquidus.  As  the  alloy  cools  from  P  to  I  its  temperature  falling  from  T  to  t,  it  still 
remains  entirely  liquid.  At  I,  temperature  t,  solidification  begins  through  the  forma- 
tion of  crystals  whose  composition  must  be  represented  by  the  point  s  at  the  inter- 
section of  the  solidus  and  of  a  horizontal  line  through  I,  for  it  is  evident  that  the  only 
crystals  that  can  be  in  equilibrium  at  the  temperature  t  with  the  liquid  of  composition 
I  must  have  the  composition  s.  To  clarify,  if  the  crystals  in  equilibrium  with  the 
liquid  I  at  the  temperature  t  have  not  the  composition  s,  then  they  must  necessarily 
contain  either  more  of  the  metal  M '  or  less  of  that  metal.  In  other  words,  their  com- 
position may  be  represented  by  the  point  x  to  the  left  of  s  or  by  the  point  y  to  its 
right.  It  is  evident  that  the  composition  of  the  crystals  forming  at  the  temperature  t 
from  a  liquid  of  composition  /  cannot  have  the  composition  x  since  the  corresponding 
point  falls  within  the  area  MLM'SM  and  since  any  point  in  this  area  cannot  represent 
the  composition  of  the  crystals  existing  at  the  corresponding  temperature  but  must 


LESSON   XXII  —  CONSTITUTION   OF   METALLIC   ALLOYS 
fl  _  fl'  ft" 


Fig.  6.  —  Diagram  showing  how  fusibility  curves  are  constructed. 


2.5  SO 

Composition. 

Fig.  7.  —  Typical  fusibility  curve  of  binary  alloys  whose  component  metals  form  solid  solution. 

represent  on  the  contrary  the  average  composition  of  the  partly  solidified  alloy,  hence 
the  crystals  forming  at  t  in  the  case  we  are  considering  cannot  have  a  composition 
represented  by  a  point  to  the  left  of  s.  Assuming  the  composition  of  these  crystals  to 


8  LESSON  XXII  —  CONSTITUTION  OF  METALLIC  ALLOYS 

be  represented  by  the  point  y  to  the  right  of  s,  then  it  is  evident  that  these  crystals 
must  have  formed  at  z,  that  is,  at  a  temperature  considerably  higher  than  t,  which 
cannot  be  since  our  alloy  does  not  begin  to  solidify  before  the  temperature  t  is  reached. 
Hence  the  composition  of  the  crystals  forming  at  t,  when  the  alloy  P  begins  to  solidify, 
cannot  be  represented  by  a  point  to  the  right  of  s.  Clearly  the  point  s  must  indicate  the 
composition  of  those  crystals.  As  the  alloy  cools  from  I  to  o,  that  is,  from  t  to  t',  the 
crystals  which  began  to  form  at  I  continue  to  grow  by  gradual  deposition  of  crystalline 
matter  progressively  richer  in  M  while  the  remaining  liquid  bath  likewise  becomes 
richer  in  M  and  consequently  more  fusible,  its  varying  composition  being  represented 
by  II'.  For  any  point  o  within  the  area  MLM'SM,  that  is,  when  the  alloy  is  in  the  proc- 
ess of  solidification,  the  composition  of  the  crystals  in  equilibrium  at  the  correspond- 
ing temperature  t'  with  the  solution  I'  is  represented  by  the  point  s'.  This  must 


Fig.  8.  —  Iron-copper  alloy.  Copper  10  per  cent. 
Magnified  125  diameters.  Heterogeneous  crys- 
talline grains.  The  dark  parts  are  richer  in 
copper,  the  lighter  parts  richer  in  iron.  (Stead.) 

necessarily  mean  that  as  the  metal  cools  from  t  to  t'  and  while  the  crystals  are  grow- 
ing, diffusion  must  necessarily  take  place  in  each  crystal  so  that  the  concentric  layers 
of  varying  composition  of  which  we  may  conceive  that  they  are  initially  composed, 
assume  the  same  composition  s',  the  crystals  being  now  homogenous.  At  s",  tem- 
perature t",  the  solidification  is  complete.  The  last  drop  of  liquid  to  solidify  had  the 
composition  I".  By  diffusion  the  crystals,  which  began  to  form  at  s  and  grew  from  s  to 
s",  that  is,  as  the  temperature  fell  from  t  to  t",  have  assumed  a  homogenous  chemical 
composition.  While  this  diffusion,  however,  needed  to  produce  homogenous  crystals, 
readily  takes  place  during  the  crystallization  of  liquid  solutions,  the  case  is  different 
with  solid  solutions.  Unless  solidification  and  subsequent  cooling  have  been  sufficiently 
slow,  solid  solutions  may  remain  heterogenous,  i.e.  the  different  layers  of  crystalline 
matter  of  which  each  crystal  is  composed  may  not  be  of  identical  composition,  the 
proportion  of  the  most  fusible  metal  increasing  from  center  to  outside  (see  Fig.  8). 
As  an  instance  of  binary  alloys  forming  solid  solutions  the  fusibility  curve  of  gold- 
platinum  alloys  is  reproduced  in  Figure  9. 

Binary  Alloys  Forming  Definite  Compounds  and  Solid  Solutions.  —  When  two 


LESSON   XXII  —  CONSTITUTION   OF   METALLIC  ALLOYS  9 

metals  unite  to  form  solid  solutions,  it  does  not  necessarily  imply  that  they  may  not 
chemically  combine  with  each  other  as  well.  Important  instances  are  known  of  metals 
forming  a  definite  chemical  compound,  which  compound  is  then  dissolved  in  the  re- 
maining excess  metal.  Two  metals  M  and  M',  for  instance,  may  unite  chemically  to 
form  the  compound  MxM'y  and  unless  the  alloy  contains  the  two  metals  exactly  in 
the  required  atomic  proportions,  the  compound  may  form  a  solid  solution  with  the 
excess  of  metal  M  or  of  metal  M',  as  the  case  may  be.  In  such  cases  the  alloys  should 
be  considered  not  as  alloys  of  two  metals  but  of  one  metal  and  of  one  chemical  com- 
pound of  two  metals,  when  the  mechanism  described  to  explain  the  solidification  of 
solid  solutions  will  be  found  applicable.  Indeed  we  may  conceive  the  existence  of 
alloys  of  two  metals  in  which  two  definite  compounds  are  formed,  mutually  soluble 
and,  therefore,  forming  solid  solutions.  Such  alloys  should  be  considered  not  as  alloys 


leoo 
noo 
jeod 

JSOO 
IfOO 

1300 

200 

/too 

IOOO 
SCO 
SCO 
TOO 


Pe,  (174-4-°) 


too  %fr- 
fy  weight 


Liquid, 


Solid,  solution, 
fAu,  +  Pt) 


O 

Fig.  9.  —  Fusibility  curve  alloys  of  gold  and  platinum.   (Desch.) 


IO         2O        3O        +O        SO         60         7O        SO        9O      fOO 

Atom,%Pt. 


of  two  pure  metals  but  of  two  chemical  compounds,  the  mechanism  of  their  solidifica- 
tion being  then  identical  to  that  of  alloys  of  two  metals. 

Binary  Alloys  whose  Component  Metals  are  Insoluble  in  Each  Other  in  the  Solid 
State.  —  If  two  metals,  although  soluble  in  the  liquid  state,  are  insoluble  when  solid,  it 
is  evident  that  on  solidification  they  must  crystallize  separately  into  distinct  crystals 
readily  distinguishable  under  the  miscroscope,  in  other  words,  that  the  solid  alloy  must 
be  an  aggregate  of  the  two  metals.  The  study  of  the  mechanism  of  the  solidification 
of  such  alloys  and  of  their  microstructure  should  receive  some  attention. 

The  typical  cooling  or  solidification  curve  of  an  alloy  of  two  metals  insoluble  in 
each  other  when  solid  is  shown  in  Figure  10.  The  curves  are  obtained  as  previously 
explained  by  observing  the  time  required  by  the  alloy  to  cool  through  successive  and 
equal  ranges  of  temperature  and  by  plotting  the  times  against  the  corresponding  falls 
of  temperature,  or  more  conveniently  and  accurately  by  the  use  of  self-registering 
pyrometers.  The  curve  of  Figure  10  will  be  seen  to  consist  of  four  parts,  namely,  AL 


10 


LESSON  XXII  —  CONSTITUTION  OF  METALLIC  ALLOYS 


indicating  a  normal  rate  of  cooling,  LL'  a  retarded  cooling,  L'S  a  stationary  tempera- 
ture, and  SB  a  normal  rate  of  cooling  to  atmospheric  temperature.  It  is  logical  to 
infer  that  AL  represents  the  uniform  cooling  of  the  liquid  alloy  and  SB  the  uneventful 
cooling  of  the  solid  metal,  L  being  the  liquidus  point,  and  S  the  solidus.  The  portion 
of  the  curve  LL'S  represents  the  solidification  of  the  alloy  as  the  temperature  falls 
from  t  to  t'.  It  will  be  observed  (1)  that  the  solidification  begins  at  L,  temperature  t, 
(2)  that  it  proceeds  from  L  to  L'  as  the  temperature  is  falling,  and  (3)  that  the  end  of 
the  solidification  takes  place  at  a  constant  temperature,  namely,  t',  as  indicated  by  the 


T   T 


Tim  e 

Fig.  10.  —  Typical  cooling  curve  of  binary  alloy  whose  component  metals  are 
insoluble  in  each  other  in  the  solid  state. 

horizontal  portion  L'S,  the  temperature  of  the  alloy  remaining  constant  during  T'-T 
seconds. 

In  Figure  11  the  solidification  curves  of  a  number  of  alloys  of  the  same  series  have 
been  constructed  as  explained  above,  the  alloys  arbitrarily  selected  containing  respec- 
tively 10,  20,  40,  60,  and  80  per  cent  of  metal  M.  It  will  be  noted  that,  with  the 
exception  of  the  alloy  containing  40  per  cent  of  M ,  each  alloy  exhibits  two  evolutions 
of  heat,  namely  an  upper  evolution,  L,  at  varying  temperatures  and  a  lower,  E,  always 
at  the  same  temperature  regardless  of  the  composition  of  the  alloy.  By  uniting  the 
upper  points  and  the  lower  ones  as  indicated  by  dotted  lines  in  Figure  11,  the  so-called 
fusibility  curve  or  equilibrium  diagram  of  that  series  of  alloys  is  obtained.  In  Figure  12 
the  fusibility  curve  only  is  represented,  the  independent  cooling  curves  used  for  its 


LESSON   XXII  —  CONSTITUTION    OF   METALLIC   ALLOYS 


11 


construction  having  been  omitted.     The  co-ordinates  are  now  composition  and  tem- 
perature.   The  curve  indicates  clearly  the  beginning  and  the  end  of  the  solidification 


IO°/o 


Fig.  11.  —  Diagram  showing  how  fusibility  curves  are  constructed. 


O 
/OO 


^o 

60 


30        4O 
7O        6O 

Composition. 


QO 


60 

ao 


/oo 
o 


Fig.  12.  —  Typical  fusibility  curve  of  binary  alloys  whose  component  metals  are  insoluble  in  each 

other  in  the  solid  state. 

of  any  alloy  of  the  series.  The  solidification  of  an  alloy  whose  composition  corresponds 
to  the  point  R,  for  instance,  and  which,  therefore,  contains  20  per  cent  of  metal  M  and 
80  per  cent  of  metal  M'  evidently  begins  at  N  and  ends  at  P.  The  fusibility  curve  is 


12  LESSON  XXII  —  CONSTITUTION  OF  METALLIC  ALLOYS 

made  up  of  three  branches,  namely,  the  two  intersecting  lines  LE  and  L'E  starting 
respectively  from  the  melting-points  of  the  two  constituent  metals  and  a  horizontal 
line  SS'  passing  by  the  point  of  intersection  E  of  the  first  two.  This  is  the  typical 
fusibility  curve  of  binary  alloys  whose  component  metals  are  insoluble  in  each  other 
in  the  solid  condition.  The  solidification  of  these  alloys  should  now  be  examined  more 
closely.  Several  features  are  obvious.  The  different  alloys  begin  to  solidify  at  differ- 
ent temperatures  according  to  their  composition.  At  first,  as  the  percentage  of  M 
increases  from  0  to  40  (a  proportio'n  arbitrarily  selected),  the  solidification  point  is 
lowered  from  L  to  E,  while  with  further  increase  of  M  from  40  to  100  per  cent,  the 
solidification  point  is  raised  from  E  to  L'.  The  solidification  of  all  alloys,  however, 
ends  at  the  same  temperature,  namely,  at  the  temperature  S.  Clearly,  LEU  is  the 
liquidus  and  SES'  the  solidus.  The  alloy  containing  40  per  cent  of  M  is  evidently  the 
most  fusible  alloy  of  the  series  since  it  remains  liquid  until  the  temperature  S  is  reached 
while  other  alloys  begin  to  solidify  at  higher  temperatures.  This  alloy  of  lowest 
melting-point  is  known  as  the  "eutectic"  alloy,  from  the  Greek  meaning  "well  melt- 
ing." It  is  evident  that,  like  pure  metals,  eutectic  alloys  solidify  at  a  constant  tempera- 
ture, namely  the  eutectic  temperature.  Many  aqueous  solutions  also  give  rise  to  the 
formation  of  solutions  of  lowest  freezing-points  called  "  cryohydrates  "  and  which  were 
at  first  supposed  to  be  true  chemical  compounds,  that  is,  hydrates  containing  salt  and 
water  molecules  in  atomic  proportions.  And,  likewise,  eutectic  alloys  were  at  first 
supposed  to  be  definite  chemical  compounds  of  the  two  metals.  They  are  now  known 
to  be  aggregates  of  these  metals  generally  very  finely  divided.  This  will  be  made  clear 
by  following  the  solidifications  of  three  alloys,  R,  R' ,  and  R"  (Fig.  12).  The  alloy  R 
contains,  according  to  the  diagram,  20  per  cent  of  the  metal  M  and  80  per  cent  of  the 
metal  M' .  Since  it  contains  less  of  the  metal  M  than  the  eutectic  alloy,  we  may  for 
convenience  refer  to  it  as  an  hypo-eutectic  alloy,  although  of  course  in  regard  to  the 
content  of  M'  it  would  be  hyper-eutectic.  In  cooling  from  R  to  N  the  alloy  remains 
liquid.  At  N  solidification  begins  through  the  formation  of  pure  crystals  of  M',  that 
is,  of  the  metal  which  is  present  in  excess  above  the  eutectic  ratio.  The  formation  of 
pure  crystals  of  M'  continues  as  the  alloy  cools  from  N  to  P  and  meanwhile  the  portion 
of  the  alloy  remaining  liquid  (we  may  call  it  the  mother  metal)  becomes  gradually 
richer  in  M ,  i.e.  it  approaches  gradually  the  composition  of  the  eutectic  alloy.  Finally 
at  P,  temperature  S,  the  remaining  liquid  has  exactly  the  eutectic  composition  and 
now  solidifies  at  a  constant  temperature,  the  solidification  temperature  of  the  eutectic 
alloy.  In  other  words,  as  the  alloy  cools  from  N  to  P  with  formation  of  pure  crystals 
of  metal  M'  the  composition  of  the  portion  of  the  alloy  remaining  liquid  varies  ac- 
cording to  the  line  NKE,  reaching  the  composition  E,  that  is,  the  eutectic  composition, 
always  at  the  same  temperature  regardless  of  the  initial  composition  of  the  alloy. 
When  the  alloy  has  cooled  to  0,  for  instance,  a  point  between  LE  and  SE,  it  is  partly 
liquid  and  partly  solid,  its  temperature  is  T  and  the  composition  of  the  liquid  portion 
is  represented  by  K,  that  is,  it  contains  30  per  cent  of  the  metal  M.  On  further  cooling 
from  0  to  P  the  composition  of  the  mother  metal  shifts  from  K  to  E. 

In  the  case  of  the  alloy  R",  containing  a  larger  proportion  of  the  metal  M  than  the 
eutectic  alloy  and  which  we  may,  therefore,  consider  to  be  hyper-eutectic,  when  it 
reaches  the  point  N'  its  solidification  begins,  pure  crystals  of  the  metal  M  being 
formed  while  the  molten  bath  becomes  gradually  richer  in  M'  gradually  approaching, 
therefore,  the  composition  of  the  eutectic  alloy,  until  at  the  temperature  S  that  com- 
position is  reached  when  the  remaining  liquid  solidifies  at  a  constant  temperature. 


LESSON   XXII  —  CONSTITUTION   OF   METALLIC   ALLOYS 


13 


Any  point  0'  situated  between  L'E  and  S'E  indicates  an  alloy  in  part  solid  and  in 
part  liquid;  its  temperature  is  T'  and  the  composition  of  the  liquid  is  represented  by 
K'.  On  cooling  from  0'  to  P'  additional  crystals  of  pure  M  are  formed,  or  those  already 
formed  continue  to  grow  while  the  composition  of  the  molten  metal  shifts  from  K'  to  E. 

Starting  with  the  alloy  R'  of  eutectic  composition,  it  remains  liquid  until  at  E, 
temperature  S,  it  solidifies  at  a  constant  temperature,  there  being  no  excess  metal  to 
be  rejected. 

Seeing  that  the  branch  LE  corresponds  to  the  formation  of  pure  crystals  of  the 
metal  M'  and  the  branch  L'E  to  the  formation  of  pure  crystals  of  the  metal  M,  the  con- 
clusion seems  irresistible  that  their  point  of  intersection  E  must  correspond  to  the 


0 


M'- 


1 


M- 


R' 


•E 


.01 


M% 
M'% 


o 
too 


a  Ac 
2O 

60 


60 


60 

Composition. 


Fig.  13.  —  Diagram  depicting  the  mechanism  of  the  solidification  of  alloys  whose  component  metals 
are  insoluble  in  each  other  in  the  solid  state. 


simultaneous  formation  of  crystals  of  metal  M  and  of  metal  M'  and  that  the  eutectic 
alloy,  therefore,  must  be  a  finely  divided  aggregate  of  M  and  M'.  In  other  words,  at 
any  point  on  the  branch  LE,  the  alloy  is  saturated  with  the  metal  M'  so  that  the  lower- 
ing of  its  temperature  must  cause  the  separation  of  M'  crystals  with  corresponding 
lowering  of  the  saturation  point,  that  is,  of  the  solidification  point  of  the  bath.  In  a 
similar  way,  at  any  point  on  the  branch  L'E,  the  alloy  is  saturated  with  M  and  a  fall 
in  its  temperature  must  result  in  the  formation  of  M  crystals  while  the  solidification 
point  of  the  portion  remaining  liquid  is  thereby  lowered.  At  E  the  alloy  is  saturated 
with  both  metals  so  that  any  attempt  at  lowering  its  temperature  must  result  in  the 
simultaneous  deposition  of  crystals  of  M  and  of  M',  and  since  the  composition  of  the 
bath  remains  the  same,  solidification  now  takes  place  at  a  constant  temperature. 
Hence  the  constitution  of  eutectic  alloys  and  the  reason  for  their  constant  freezing 
temperature. 

It  has  been  attempted  in  Figure  13,  to  depict  graphically  the  mechanism  of  the 


14 


LESSON   XXII  —  CONSTITUTION   OF   METALLIC   ALLOYS 


solidification  of  hypo-eutectic,  eutectic,  and  hyper-eutectic  alloys.     As  diagrams  of  this 

kind  have  already  been  used  in  these  lessons  the  present  one  will  be  readily  understood. 

When  the  alloy  R,  for  instance,  reaches  the  point  N,  crystals  of  M'  form,  their 

.8 
is 

io 
O 

I 

o 
O 


I 

I 


M  °/0 
M'°/0 


IOO 

75. 

SO. 

a                                                                              a' 

Metaf 
M' 

h 

/- 

^        ^,                       ft 

f                                            ^ 

^  -Eutecfic—         ^^ 
M  +  M'  =                  ^^ 

fefa/ 
M 

& 

2.5. 

/- 

;{.           „ 

>  

M.       =c 

i  .  

=^,=^==^-^  ~-  ,    

O  '  —                 ^N 

O 

IOO 


2O 

<3O  QO 

Chemical    compos/  f  /on. 


6O 

-40 


<30 

20 


/OO 
o 


Fig.  14.  —  Diagram  showing  the  structural  composition  of  binary  alloys  whose  component  metals 
are  insoluble  in  each  other  in  the  solid  state. 


%M     o 
%  M '  /oo 


Chemical    composition. 


Fig.  15.  —  Diagram  showing  the  fusibility  curve  and  the  structural  composition  of  binary  alloys 
whose  component  metals  are  insoluble  in  each  other  in  the  solid  state. 

formation  as  the  metal  cools  from  N  to  P  being  represented  by  the  area  NPP'.  At  P 
the  residua!  liquid,  now  of  eutectic  composition,  solidifies  at  a  constant  temperature. 
At  any  point  0  the  composition  of  the  portion  still  liquid  is  represented  by  A'.  The 
completely  solidified  metal,  will  be  made  up  of  ob  per  cent  of  metal  M  and  be  per  cent 
of  eutectic  alloy. 


LESSON   XXII  —  CONSTITUTION   OF   METALLIC   ALLOYS 


15 


It  will  not  be  necessary  to  describe  at  greater  length  the  graphical  representation 
of  the  solidification  of  the  alloys  R'  and  R". 

The  structural  composition  of  alloys  of  two  metals  completelyinsoluble  in  each  other 
in  the  solid  condition  may  conveniently  be  represented  graphically  as  shown  in  Figure 


0 

• 
E 
3 

1 

§326 

228 

Sb^ 

nn 

632 

600 

500 
400 
300 
200 

^ 

^ 

x"*^ 

^ 

s^ 

—  —  - 

- 

Pb 
\1 

^ 

^ 

^ 

X 

s^ 

S* 

10 


80 


90       100 


20        30         40        50        60         70 
Percentages  of  antimony  by  weight 

Fig.  16.  —  Fusibility  curve  of  alloys  of  lead  and  antimony.    (Roland-Gosselin.) 


Fig.  17.  —  Typical  structures  of  alloys  whose  component  metals 
are  insoluble  in  each  other  in  the  solid  state,  x,  excess  metal 
M'  and  eutectic;  y,  eutectic;  z,  excess  metal  M  and  eutectic. 
(Gulliver.) 

14.  The  interpretation  of  this  diagram  is  obvious.  An  alloy  containing  20  per  cent 
of  the  metal  M,  for  instance,  will  be  made  up  of  ab  per  cent  of  M'  and  be  per  cent  of  eu- 
tectic alloy.  With  40  per  cent  of  M  the  alloy  is  wholly  of  eutectic  composition  while  with 
80  per  cent  of  M ,  for  instance,  it  contains  a'b'  of  M  crystals  and  b'c'  per  cent  of  eutectic. 


16  LESSON  XXII  —  CONSTITUTION  OF  METALLIC  ALLOYS 

The  composition  diagram  may  with  advantage  be  combined  with  the  equilibrium 
diagram  as  it  has  been  done  in  Figure  15.  Taking  an  alloy  whose  composition  is 
represented  by  R,  for  instance  (20  per  cent  M  and  80  per  cent  M'),  it  is  seen  (1)  that 
it  begins  to  solidify -at  N,  (2)  that  as  it  cools  from  NtoP  pure  crystals  of  M'  are  formed, 
(3)  that  the  percentage  of  M'  thus  liberated  is  represented  by  PK,  (4)  that  at  P  the 
remaining  molten  alloy  now  of  eutectic  composition  solidifies,  and  (5)  that  KO  repre- 
sents the  percentage  of  this  eutectic  alloy. 

The  graphical  method  used  in  these  lessons  for  representing  the  structural  com- 
position of  alloys  was  first  suggested  by  the  author  in  1896.  It  has  since  been  widely 
used  and  Tammann  employs  it  to  represent  the  heat  liberated  by  the  solidification,  of 
the  eutectic  or  the  time  taken  for  its  solidification,  it  being  evident  that  both  heat 
and  time  must  necessarily  be  proportional  to  the  amount  of  eutectic  alloy  formed.  In 


T.  "A  -''— ~ 


;-?>  v= 


Fig.  18.  —  Eutectic  alloy  of  bismuth  and   tin. 
Magnified  200  diameters.     (Desch.) 

Figure  15,  therefore,  the  vertical  distances  of  the  shaded  area  are  proportional  to  the 
times  during  which  the  temperature  of  the  alloys  remained  constant  while  the  residual 
baths  of  eutectic  composition  were  solidifying.  The  fusibility  curve  of  lead-antimony 
alloys  is  reproduced  in  Figure  16  as  an  instance  of  alloys  whose  component  metals  are 
entirely  insoluble  in  each  other  after  solidification. 

From  the  foregoing  it  appears  that  solidified  alloys  of  two  metals  insoluble  in  each 
other  are  aggregates  of  these  two  metals  and  that  three  types  of  structure  are  to  be 
expected  (1)  the  structure  of  hypo-eutectic  alloys  composed  of  crystals  or  crystalline 
grains  or  particles  of  one  metal  embedded  in,  or  surrounded  by,  some  eutectic  alloy, 
(2)  the  structure  of  hyper-eutectic  alloys  consisting  of  crystalline  particles  of  the 
other  metal  and  of  eutectic  alloy,  and  (3)  the  structure  of  eutectic  alloys  consisting 
of  a  finely  divided  aggregate  of  minute  particles  of  both  metals.  These  three  types 
are  represented  diagramatically  in  Figure  17.  Eutectic  alloys  are  often  made  up  of 
very  thin  alternate  and  parallel  plates  or  lamellae  of  each  of  the  two  constituents, 
but  in  some  cases  they  consist  of  rounded  or  elongated  particles  of  one  of  the  con- 
stituents embedded  in  a  matrix  of  the  other  constituents.  The  structures  of  some 


LESSON   XXII  —  CONSTITUTION   OF   METALLIC   ALLOYS 


17 


eutectic  alloys  are  shown  in  Figures  18  to  20.     It  will  be  noted  that  the  constituents 
of  eutectic  alloys  are  not  always  pure  metals. 

Binary  Alloys  whose  Component  Metals  are  Partially  Soluble  in  each  Other  when 
Solid.  —  Metals  are  seldom  absolutely  insoluble  in  each  other  when  solid,  each  metal 


Fig.  19.  —  Eutectic  (eutectoid)  alloy  of  iron  and  FesC.  Pearlite.  Magni- 
fied 1000  diameters.     (Law.) 


Fig.  20.  —  Eutectic  alloy  of  SnCu4  and  Cu3P.     Magnified  1000 
diameters.     (Law.) 

in  the  majority  of  cases  being  capable  of  retaining  in  solid  solution  a  small  percentage 
of  the  other  metal.  The  modifications  which  such  partial  solubility  introduces  in  the 
fusibility  curve  should  be  considered.  Let  us  suppose  two  metals  M  and  M'  and  let 
us  assume  that  the  metal  M'  is  capable  of  retaining  in  solution  immediately  after 


18 


LESSON  XXII  —  CONSTITUTION  OF  METALLIC  ALLOYS 


solidification,  i.e.  at  the  eutectic  temperature,  5  per  cent  of  the  metal  M  and  that  the 
metal  M  can  retain  in  solid  solution  10  per  cent  of  the  metal  M'.  The  fusibility  curve 
of  these  alloys  constructed  in  the  usual  way  will  have  the  appearance  indicated  in 


o 

i 

<D 

I 


M  %      O 

M'%  (00  Q7  SO  60 

Compos!  ti'on . 

Fig.  21.  —  Typical  fusibility  curve  of  alloys  whose  component  metals  are  partially  soluble  in  each 

other  in  the  solid  state. 


loo 


.0 

i  So/  id  Solution             /      \ 

.                        Solid   solution 

"> 

M  -t-  3%>  fVI             / 

YK.                    A^  +  G  °/o  M 

I 

^                     M\ 

TfJK                                                                                           C 

g» 

o                            yf  HI 

N.                                                                                                               »  " 

o 

"i"                    A     Eu,+ec 

+  ''c      IfflK                               i 

°  £°- 

f\1                                                                JT<                                                                            ' 

>oM          fllTiv                          ^ 

e 

A     M  +60> 

'°  M'i             lltflfk                     " 

^ 

'A          A 

"TV                                      *  *** 

0 

o'               yf 

TK                       >s> 

i 

^o  '          y*f 

'  TV 

V. 

1         yl^ 

Phv                     ^0 

$ 

Lri 

|j|K 

o 

ullll 

1  i  II  1  1  1  1  11  1  liTv 

M  °/o    O    3                     2O                          *4O                          6O                         <3O               94/OO 

M'°/o  100  91                   dO                            6O                           40                          20                   €          O 

i~f~ion 

Fig.  22.  —  Diagram  showing  the  structural  composition  of  binary  alloys  whose  component 
are  partially  soluble  in  each  other  in  the  solid  state. 


met;d 


Figure  21.  By  comparing  it  with  the  fusibility  curve  of  metals  entirely  insoluble 
(Fig.  12)  it  will  be  noted  that  the  only  differences  between  them  are  (1)  that  the 
eutectic  line  SES'  instead  of  extending  over  the  whole  length  of  the  diagram  now  stops 
at  the  points  S  and  S'  corresponding  respectively  to  5  per  cent  of  metal  M  and  to  10 


LESSON  XXII  —  CONSTITUTION   OF   METALLIC   ALLOYS 


19 


per  cent  of  metal  M',  and  (2)  the  introduction  of  the  branches  SA  and  S'B  indicates  the 
changes  of  solubility  of  the  metals  as  the  alloys  cool  from  the  eutectic  to  atmospheric 
temperature.  These  curves  show  that  at  atmospheric  temperature  M'  retains  in 
solution  3  per  cent  of  M  and  that  M  retains  in  solution  6  per  cent  of  M' .  LEU  is 
the  liquidus,  LSES'L'  the  solidus.  Clearly,  any  alloy  containing  less  than  5  per  cent 
of  the  metal  M  or  less  than  10  per  cent  of  the  metal  M'  solidifies  as  a  solid  solution, 
the  diagram  showing  the  absence  of  eutectic  alloys.  In  other  words,  alloys  contain- 
ing from  0  to  5  per  cent  of  M  may  be  considered  as  alloys  of  M'  and  of  a  solid  solution 
of  M'  plus  5  per  cent  of  M,  LIP  being  the  liquidus  and  LS  the  solidus  of  such  alloys 
and  their  solidification  taking  place  as  explained  in  the  case  of  any  two  metals  form- 
ing solid  solutions.  And,  likewise,  alloys  with  less  than  10  per  cent  of  M'  may  be  re- 
garded as  alloys  of  the  metal  M  and  of  a  solid  solution  of  M  plus  10  per  cent  of  M', 
their  liquidus  being  represented  by  L'M  and  their  solidus  by  L'S'.  For  all  alloys 
containing  between  5  and  90  per  cent  of  M  the  diagram  shows  that  a  eutectic  alloy  is 
formed  and  that  the  mechanism  of  the  solidification  is  apparently  the  same  as  the  one 


'Co 

1100  i 


so        30        •fa       so     60    jo    go   90  100% Cu 

' 


IOOO- 

900 

coo 
700 
600 
£00 


lost", 


962 


Solid-  solution.  A.  -f- 
Solid.  soUiticn  B 


\   B 


10         20        3O       40         SO        SO         7O         80       9O        IOO 

Atom..°/aCu. 

Fig.  23.  —  Fusibility  curve  of  alloys  of  silver  and  copper. 
(Desch.) 

described  in  the  case  of  alloys  of  insoluble  metals.  It  should  be  noted,  however,  that 
the  two  components  of  these  alloys  are  no  longer  the  pure  metals  but  two  solid  solu- 
tions, namely,  a  solution  of  M'  containing  5  per  cent  of  M  and  a  solution  of  M  con- 
taining 10  per  cent  of  M',  one  of  these  solid  solutions,  therefore,  crystallizing  when  the 
temperature  of  the  alloy  reaches  any  point  on  the  lines  LEL'  and  the  eutectic  alloy 
being  made  up  of  a  fine  conglomerate  of  these  two  solid  solutions.  To  clarify  let  us 
consider  an  alloy  represented  by  the  point  R,  Figure  21.  At  N  its  solidification  begins, 
crystals  of  a  solid  solution  of  M'  containing  5  per  cent  of  M  being  formed.  At  P  the 
residual  molten  mass,  having  reached  the  eutectic  ratio,  crystallizes  at  a  constant 
temperature  as  a  fine  aggregate  of  the  two  solid  solutions.  Since  the  mutual  solubili- 
ties of  the  metals  M  and  M'  decrease,  however,  as  the  alloy  cools  below  the  eutectic 
temperature,  it  is  evident  that  each  crystal  must  undergo  a  gradual  transformation. 
These  transformations  are  indicated  by  the  branches  SA  and  S'B.  After  the 
solidification  of  the  eutectic,  all  alloys  are  aggregates  of  the  two  solid  solutions  whose 
compositions  are  represented  by  the  points  S  and  S',  that  is,  in  the  case  under  con- 
sideration, arbitrarily,  M'  plus  5  per  cent  of  M  and  M  plus  10  per  cent  of  M'.  At  atmos- 
pheric temperature  all  alloys  are  aggregates  of  two  solid  solutions  whose  compositions 
correspond  to  the  points  A  and  B,  that  is  in  the  case  considered  M'  plus  3  per  cent  of  M 


20 


LESSON   XXII  —  CONSTITUTION   OF   METALLIC   ALLOYS 


and  M  plus  6  per  cent  of  M'.  At  any  point  below  the  eutectic  line,  the  corresponding 
alloy  is  an  aggregate  of  two  solid  solutions  whose  compositions  are  indicated  by  the 
corresponding  points  on  the  branches  SA  and  S'B.  At  C,  for  instance,  in  case  of  alloy  7?, 
the  structure  is  composed  of  free  crystals  of  solid  solution  D  and  of  eutectic,  the  com- 


Fig.  24.  —  Silver-copper  alloy.  Copper  15  per  cent. 
Magnified  600  diameters.  Dark  constituent  is 
silver  containing  a  little  copper.  (Osmond.) 


Fig.  25.  —  Silver-copper  alloy  eutectic. 
Copper  28  per  cent.  Magnified  600 
diameters.  (Osmond.) 


Fig.  26.  —  Silver-copper  alloy.  Copper  65  per  cent. 
Magnified  600  diameters.  Light  constituent  is 
copper  containing  a  little  silver.  (Osmond.) 


ponents  of  which  are  solid  solution  D  and  solid  solution  F.  In  other  words,  as  the  alloy 
cools  from  the  eutectic  line  to  atmospheric  temperature,  the  composition  of  its  two 
constituents  shifts  respectively  from  S  to  A  and  from  8'  to  B. 

The  structural  composition  of  alloys  whose  component  metals  are  partially  soluble 
when  solid  may  be  represented  in  the  usual  way  as  shown  in  Figure  22.    Its  interpre- 


LESSON  XXII  —  CONSTITUTION  OF  METALLIC  ALLOYS  21 

tation  does  not  call  for  further  elaboration.  Between  0  and  3  per  cent  of  M  and 
between  0  and  6  per  cent  of  M',  solid  solutions  are  formed  of  corresponding  composi- 
tions. Between  3  and  40  per  cent  of  M ,  the  free  solid  solution  formed  is  saturated  with 
M,  and  between  40  and  94  per  cent  of  M  it  is  saturated  with  M',  while  the  eutectic 
is  made  up  of  the  two  saturated  solutions.  The  fusibility  curve  of  silver-copper  alloys 
is  shown  in  Figure  23  as  an  example  of  alloys  whose  components  remain  partly  soluble 
in  each  other  after  solidification  and  typical  structures  of  these  alloys  are  given  in 
Figures  24  to  26. 

Examination 

Describe  briefly  the  mechanism  of  the  solidification  of  binary  alloys  (1)  completely 
soluble,  (2)  completely  insoluble,  and  (3)  partly  soluble  in  each  other  in  the  solid  state. 


LESSON  XXIII 


EQUILIBRIUM  DIAGRAM  OF  IRON-CARBON  ALLOYS 

Fusibility  Curve  of  Iron-Carbon  Alloys.  —  Steel  and  cast  iron  are  essentially  alloys 
of  iron  and  carbon,  and  their  fusibility  curve  or  equilibrium  diagram  may  be  con- 
structed as  in  the  case  of  any  binary  alloy,  namely,  by  determining  the  independent 
cooling  curves  of  a  number  of  alloys  of  the  series  and  plotting  the  evolutions  of  heat 

/S"oo 


looo 
Percen  f  C     O 

PercenfFe,C  O 


l.O       t.7  2.O  v3.0  4.O  4.3        5O  6O        6.67 

IS  vSO  -4S  GO  7-5"  9O         IOO 

Fig.  1.  — •  Fu-sibility  curve  of  iron-carbon  alloys. 


noted  against  the  corresponding  temperatures.    The  resulting  curve  is  shown  in  the 
diagram  of  Figure  1. 

While  it  is  not  generally  possible  for  molten  iron  to  dissolve  more  than  5  per  cent 
of  carbon,  the  diagram  has  been  constructed  so  as  to  include  a  percentage  of  carbon  up 
to  6.67  per  cent  which  corresponds  to  100  per  cent  Fe3C.  That  portion  of  it,  however, 
corresponding  to  more  than  5  per  cent  of  carbon,  has  been  drawn  in  dotted  lines. 
The  complete  equilibrium  diagram  should  include  the  evolutions  of  heat  occurring 
after  solidification,  known  as  the  critical  points,  which  have  been  fully  described  in 
these  lessons,  but  they  are  purposely  left  out  of  the  diagram  of  Figure  1,  it  being 
desired  first  to  confine  our  attention  to  the  mechanism  of  the  solidification  of  iron- 

1 


LESSON   XXIII  —  EQUILIBRIUM   DIAGRAM   OF   IRON-CARBON   ALLOYS 


carbon  alloys.  In  view  of  the  explanation  of  the  meaning  of  fusibility  curves  given 
in  Lesson  XXII,  it  will  be  evident  that  iron  and  carbon  alloys  are  partially  soluble  in 
each  other  when  solid,  that  LEU  is  their  liquidus  and  LSS'  their  solidus  and  that  the 
point  E  indicates  the  formation  of  a  eutectic  alloy.  As  carbon  increases  from  0  to 
about  1.70  per  cent,  the  alloys  solidify  as  solid  solutions  of  corresponding  carbon 
content,  LA  being  the  liquidus  and  LS  the  solidus.  These  solid  solutions  considered  as 
microscopical  constituents  are  called  austenite.  The  solidification  of  alloys  containing 
between  1.7  and  4.3  per  cent  carbon  begins  when  their  temperature  reaches  the  line 
LE,  crystals  of  a  solid  solution  containing  1.70  per  cent  carbon  (sometimes  called 
saturated  austenite),  being  then  formed  which  keep  on  growing  until  the  line  SS', 
temperature  1130  deg.,  is  reached  when,  clearly,  a  eutectic  is  formed  composed  of 


Hypo  -evtecf/  c   a/  toys                        / 

~/yper-  eufecTic 

too 

evrectoid                                     Hyper-  e  u  te  c  tte  id  a  1  toys 

Pro-eutectic             /     \ 

Pro  -eut~ect~i  c 

-Saturated             / 

\    Cemenf/fe. 

C 

Austenite           A 

-5    15 

A 

k 

o 

& 

A\\  I 

c    .,,      ,,.                                   A\\£vfec-t 
Solid  solution                              / 

"\ 
<c      \ 

O       ,r>» 

(Austen  ite)                                ASofurated  At. 

istenite   V 

i,    SO 

A     •+  Cement/ 

te)            \ 

"o 

M 

* 

e 

/ 

V. 

•K 

/\ 

\ 

54f 

i 

\ 

•V, 

/ 

V 

^ 

A 

yK 

o- 

M\\ 

°/oC       O                  '/.O                 2.0                ^.0                4.O 

SO                €>.O          G.G7 

%fc3C  0                  /5                 30                45               60                 7S                90           100 

Fig.  2.  —  Structural  composition  of  iron-carbon  alloys  immediately  after  their  solidification. 

that  solid  solution  and  of  another  constituent.  The  nature  of  the  other  constituent 
present  in  the  eutectic  alloy  formed  during  the  solidification  of  iron-carbon  alloys  has 
been  in  dispute.  It  seems  at  first  natural  to  infer  that  elemental  carbon,  i.e.  graphite, 
is  that  constituent,  in  which  case  the  eutectic  alloy  would  be  made  up  of  minute  crys- 
talline particles  alternately  of  saturated  austenite  and  of  graphite.  Many  evidences, 
however,  point  to  carbon  being  dissolved  in  molten  iron  as  the  carbide  Fe3C  (cement  ite) 
and  to  its  always  solidifying  as  Fe3C,  although,  as  later  explained,  it  may  break  up 
into  iron  and  graphite  (FesC  =  3Fe  +  C)  immediately  after  its  solidification.  If  this 
hypothesis  be  correct,  it  follows  that  the  eutectic  alloy  must  be  a  mechanical  mixture 
of  minute  particles  of  saturated  austenite  and  of  Fe3C.  It  would  seem  at  first  as  if 
the  microscopical  examination  of  alloys  of  suitable  compositions  should  readily  reveal 
the  nature  of  the  eutectic  alloy.  It  will  soon  be  seen,  however,  that  both  cementite 
and  graphite  are  generally  found  in  solidified  eutectiferous  alloys,  the  microscopical 
test  leaving  us  in  doubt  as  to  which  of  the  two  constituents  formed  first.  In  the 


LESSON   XXIII  —  EQUILIBRIUM   DIAGRAM   OF   IRON-CARBON   ALLOYS          3 

author's  opinion  it  seems  more  probable  that  when  an  iron-carbon  alloy  containing 
more  than  some  1.7  per  cent  carbon  solidifies,  a  eutectic  of  saturated  austenite  and  of 
cementite  is  always  produced,  or  in  other  words,  that  in  the  diagram  of  Figure  1  the 
curve  L'E  indicates  the  crystallization  of  cementite  and  not  of  graphite.  The  opposite 
view  will  be  considered  later.  Let  us  now  follow  the  solidification  of  four  typical 
alloys,  namely,  R,  R',  R",  R'",  containing  respectively  1  per  cent,  3  per  cent,  4.3  per 
cent,  and  4.8  per  cent  of  carbon,  the  first  two  being,  therefore,  hypo-eutectic  alloys, 
the  third  the  eutectic  alloy,  and  the  fourth  a  hyper-eutectic  alloy.  As  the  alloy  R 
cools,  it  begins  to  solidify  at  M  through  the  formation  of  crystals  of  a  solid  solution 
the  composition  of  which  is  represented  by  the  point  T  on  the  solidus.  On  cooling  from 
M  to  N  these  crystals  grow  through  successive  additions  of  crystalline  matter,  the 
composition  of  which  varies  from  T  to  N  while  the  composition  of  the  molten  bath 
shifts  from  M  to  U,  the  last  drop  solidifying  having  the  composition  U.  As  soon  as  the 
crystalline  matter  is  deposited,  however,  at  least  if  time  be  given,  diffusion  takes  place 
through  each  crystal  so  that  finally  they  are  chemically  homogeneous  and  of  com- 
position .V,  the  completely  solidified  metal  being  composed  of  homogeneous  crystalline 
grains  of  austenite  containing  one  per  cent  carbon. 

At  any  temperature  V  between  the  solidus  and  the  liquidus  the  crystals  in  equilib- 
rium with  the  molten  metal  must  have  the  composition  Q.  It  may  at  least  be  con- 
ceived that  if  the  cooling  through  and  below  the  solidification  period  be  rapid,  the 
crystalline  grains  of  austenite  will  not  be  chemically  homogeneous,  complete  diffusion 
having  been  prevented. 

In  the  case  of  an  alloy  whose  composition  is  represented  by  the  point  R'  in  the 
diagram,  it  begins  to  solidify  at  M'  (some  1230  degrees  C.),  when  crystals  of  iron 
containing  1.70  per  cent  of  carbon  (saturated  austenite)  begin  to  form,  the  composition 
of  the  molten  metal  meanwhile  shifting  from  M'  to  E.  At  0',  temperature  1130,  the 
residual  molten  mass  has  reached  the  eutectic  composition  and  now  solidifies  at  a 
(•(jnstant  temperature,  namely,  the  eutectic  temperature,  the  completely  solid  metal 
being  made  up  of  crystalline  grains  of  saturated  austenite  surrounded  bya  eutectic  alloy 
composed  of  minute  crystals  of  saturated  austenite  and  minute  crystals  of  cementite. 

The  alloy  R"  has  the  eutectic  composition  (4.3  per  cent  carbon).  It  remains 
liquid  until  its  temperature  falls  to  1130  deg.  C.  when  it  solidifies  at  a  constant 
temperature  after  the  fashion  of  eutectic  alloys. 

The  alloy  R'"  contains  more  carbon  than  the  eutectic  alloy.  On  reaching  its 
solidification  point  M'"  crystals  of  cementite  begin  to  form,  increasing  in  size  as  the 
metal  cools  from  M'"  to  0'"  while  the  composition  of  the  bath  shifts  from  M"'  to  E. 
At  0'"  the  residual  molten  mass  having  now  the  composition  of  the  eutectic  alloy, 
freezes  at  a  constant  temperature,  the  completely  solidified  alloy  being  made  up  of 
crystals  of  cementite  embedded  in  a  ground  mass  consisting  of  the  eutectic  alloy. 

Structural  Composition  of  Iron-Carbon  Alloys  Immediately  after  Solidification.  — 
The  structural  composition  of  iron-carbon  alloys  immediately  after  their  solidification, 
assuming  that  Fe3C  and  not  graphite  forms,  may  be  represented  in  the  usual  way  by 
the  diagram  of  Figure  2.  The  diagram  clearly  shows  (1)  that  alloys  containing  less 
than  1.70  per  cent  of  carbon  are  made  up  wholly  of  solid  solutions,  (2)  that  alloys 
containing  between  1.70  and  4.3  per  cent  carbon  are  made  up  of  decreasing  propor- 
tions of  saturated  austenite  and  increasing  proportions  of  eutectic,  (3)  that  alloys 
containing  exactly  4.3  per  cent  of  carbon  are  composed  entirely  of  eutectic,  and  (4)  that 
alloys  containing  more  than  4.3  per  cent  carbon  contain  an  increasing  amount  of  free 


LESSON  XXIII  —  EQUILIBRIUM   DIAGRAM  OF  IRON-CARBON  ALLOYS 


cementite,  and  decreasing  amount  of  eutectic  the  latter  disappearing  altogether  when 
the  metal  contains  6.67  per  cent  carbon. 

A  modified  form  of  the  structural  composition  diagram  may  profitably  be  combined 

ISOO 


/oo 


o 

I 

O 


o 


-50 


O 
O 


c 


/=> 
3 


H 


F" 
6.67 


F>erceni~    Carbon. 

Fig.  3.  —  Fusibility  curve  and  structural  composition  diagram  of  iron-carbon  alloys. 

with  the  equilibrium  diagram  as  shown  in  Figure  3.  Its  interpretation  should  be 
obvious.  The  area  ABCD  represents  the  structural  composition  of  all  alloys  contain- 
ing less  than  1.7  per  cent  of  carbon,  and  shows  that  they  are  made  up  of  100  per  cent 


LESSON  XXIII  —  EQUILIBRIUM   DIAGRAM   OF   IRON-CARBON  ALLOYS 


of  austenite.  By  dividing  this  area  by  the  line  AC,  however,  it  is  further  shown  graphi- 
cally that  the  composition  of  the  austenite  of  these  alloys  varies,  and  that  they  may  be 
considered  as  being  made  up  of  ABC  saturated  austenite  diluted  by  ADC  pure  iron, 
clearly  indicating  that  with  0  carbon  the  alloy  is  entirely  made  up  of  iron  and  with 
1.7  per  cent  carbon  entirely  of  saturated  austenite.  The  area  BCH  represents  the 
proportion  of  free  (pro-eutectic)  saturated  austenite  formed  during  the  solidification 
of  any  alloy  containing  between  1.70  and  4.3  per  cent  carbon.  The  area  BEFH  rep- 
resents the  proportion  of  eutectic  present  in  any  alloy  containing  more  than  1.7  per 
cent  of  carbon.  By  dividing  this  area  in  two  portions,  moreover,  by  means  of  the  line 
BF  we  show  graphically  the  relative  proportions  of  saturated  austenite  and  of  cemen- 
tite in  the  eutectic.  Finally,  the  area  EOF  indicates  the  percentage  of  pro-eutectic 


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PercentC    0                1.0             20             JO            <4.O             5O             6O       667 

Percent  fe  Co               is            do            45           6O              75            90       IOO 

Fig.  4.  —  Iron-graphite  fusibility  curve  of  iron-carbon  alloys. 

ccmentite  in  any  alloy  containing  more  than  4.3  per  cent  carbon.  To  clarify,  let  us 
consider  the  alloy  of  composition  R  (3  per  cent  carbon).  As  it  cools  from  M  to  N  an 
amount  of  saturated  austenite  crystallizes,  represented  in  percentage  by  the  line  OP 
of  the  structural  composition  diagram.  At  ./V  an  amount  of  eutectic  alloy  is  formed, 
represented  by  the  distance  KO  made  up  of  K  L  per  cent  of  cementite  and  LO  per  cent 
of  saturated  austenite. 

The  percentage  of  cementite  and  of  saturated  austenite  in  the  eutectic  may  be 
readily  calculated  by  solving  the  equations 

(1)  A  +  Cm  =  100 


in  which  A  represents  the  percentage  of  austenite,  and  Cm  the  percentage  of  cementite 
in  the  eutectic  alloy  and  which  express  the  facts  that  the  carbon  present  in  the  eutectic 


6          LESSON  XXIII  —  EQUILIBRIUM   DIAGRAM  OF  IRON-CARBON  ALLOYS 

(4.3  per  cent)  is  divided  between  the  austenite  and  the  cementite,  the  former  contain- 
ing 1.7  per  cent  carbon,  the  latter  6.67  per  cent.  The  resolution  of  these  equations 
shows  that  the  eutectic  alloy  contains  nearly  47.7  per  cent  of  saturated  austenite  and 
52.3  per  cent  of  cementite.  The  structural  composition  of  any  iron-carbon  alloy 
immediately  after  its  solidification  may  likewise  be  readily  calculated.  If  it  contains 
less  than  1.70  per  cent  carbon  it  is  entirely  made  up  of  austenite.  If  more  highly 
carburized,  two  cases  are  to  be  considered:  (1)  the  alloy  contains  between  1.7  and  4.3 
per  cent  carbon  when  it  is  made  up  of  saturated  austenite  (A)  and  of  eutectic  (E)  and 
(2)  the  alloy  contains  between  4.3  per  cent  and  6.67  per  cent  carbon  when  it  is  com- 
posed of  cementite  (Cm)  and  of  saturated  austenite  (A). 

/SO  a. 


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/300. 


-k 


I2OO  . 


I- 


1100 


IOOO 

Percent  C          O 
Percent  Fe^C      O 

Fig.  5.  —  Combined  graphite-cementite  fusibility  curves  of  iron-carbon  alloys. 

In  the  first  instance  the  two  following  equations 

(1)  A  +  E  =  100 


will  give  the  values  of  A  and  E  for  any  known  carbon  content  (C)  while  in  the  latter 
case  the  equations 

(1)  Cm  +  E  =  100 


will  likewise  give  the  values  of  Cm  and  E. 

An  alloy,  for  instance,  containing  3  per  cent  of  carbon  will  be  found  to  contain  50 
per  cent  of  eutectic  and  50  per  cent  of  saturated  austenite,  while  an  alloy  with  5  per 
cent  carbon  is  composed  of  70.5  per  cent  of  eutectic  and  29.5  per  cent  free  cementite. 


LESSON   XXIII  —  EQUILIBRIUM   DIAGRAM   OF   IRON-CARBON   ALLOYS          7 

Iron-Graphite  Fusibility  Curve.  —  It  has  already  been  mentioned  that  some 
writers  claim  that  graphite  instead  of  cementite  may,  and  if  time  be  given  does,  form 
on  solidification;  in  other  words,  that  the  eutectic  alloy  may  be  composed  of  saturated 
austenite  and  graphite,  and  that  free  graphite  may  separate  during  the  solidification 
of  alloys  containing  more  than  4.3  per  cent  carbon.  The  diagram  interpreting  this 
assumption  which  may  be  called  the  iron-graphite  fusibility  curve  is  shown  in  Figure  4. 
It  will  be  seen  to  be  similar  to  the  iron-cementite  diagram  (Fig.  1). 

Combined  Graphite-Cementite  Diagram.  —  Recognizing  the  possibility  of  the 
formation  of  a  graphite-austenite  eutectic  and  of  a  cementite-austenite  eutectic  ac- 
cording to  the  rate  of  cooling,  some  writers,  notably  Charpy  and  Grenet,  Heyn,  and 
Benedicks,  recommend  the  use  of  double  solidification  curves  ifTtftc  equilibrium  dia- 
gram of  iron-carbon  alloys.  These  double  curves  are  shown  in  Figure  5,  the  dotted 
lines  referring  to  the  austenite-graphite  system.  It  will  be  noted  that  free  graphite 
and  the  graphite-austenite  eutectic  form  respectively  at  temperatures  slightly  higher 
than  those  at  which  free  cementite  and  the  cementite-austenite  eutectic  form,  the 
solidification  of  the  latter  constituents  being  regarded  as  due  to  surfusion  or  under- 
cooling. It  is  accordingly  believed  that  only  the  iron-graphite  system  is  stable,  the 
iron-cementite  system  being  "metastable."  Our  reasons  for  believing  that  graphite 
and  not  cementite  is  the  final  condition  to  be  assumed  by  carbon  are  based  on  repeated 
and  concordant  observations  that  any  condition  promoting  stable  equilibrium  results 
in  the  transportation  of  cementite  into  graphite  as,  for  instance,  very  slow  cooling 
during  and  below  solidification  or  long  exposure  of  cementite  (as  in  the  manufacture 
of  malleable  cast-iron  castings)  to  a  high  temperature,  while  on  the  contrary,  treat- 
ments opposing  equilibrium,  such  as  quick  cooling,  always  result  in  the  formation  or 
retention  of  cementite.  Roozeboom,  when  he  first  took  up  the  study  of  the  iron- 
carbon  diagram,  believed  that  cementite  was  the  final  stable  condition  of  carbon,  any 
graphite  having  formed  during  solidification  combining  with  iron  at  some  1000  degrees 
C.  to  form  cementite.  The  error  of  this  view  soon  became  apparent,  however,  to 
Roozeboom  himself. 

Graphitizing  of  Cementite.  —  Although  recognizing  the  fact  that  graphite  and  not 
cementite  must  be  the  final  condition  assumed  by  carbon,  the  author  believes  with 
some  other  observers  that  graphite  never  forms  directly  as  iron-carbon  alloys  solidify, 
its  occurrence  always  resulting  from  the  breaking  up  of  cementite  according  to  the 
reaction 

Fe3C  =  3Fe  +  C 

from  which  it  would  follow  that  the  iron-graphite  fusibility  curve  need  not  be  in- 
cluded in  the  equilibrium  diagram.  Even  those  who  believe  in  the  possibility  of  the 
direct  formation  of  graphite  do  not  deny  that  cementite  is  the  constituent  which 
generally  forms  first  on  solidification;  they  state  that  the  separation  of  graphite  from 
molten  iron  is  possible  only  in  the  case  of  very  slow  cooling.  As  a  matter  of  fact,  how- 
ever, they  offer  no  conclusive  evidences  that  such  separation  ever  takes  place.  From 
the  formation  of  "kish,"  that  is,  of  graphite  floating  on  the  surface  of  a  ladleful  of 
molten  cast  iron,  it  does  not  follow  that  such  graphite  formation  was  not  preceded  by 
the  formation  of  cementite.  If,  on  very  slow  cooling,  graphite  separated  directly  from 
molten  iron,  the  bulk  of  it  at  least  should  rise  to  the  top  of  the  molten  bath  and  the 
solidified  mass  should  be  found  much  richer  in  graphite  near  its  surface  than  at  some 
distance  from  it.  The  author  does  not  understand  such  heterogeneity  in  the  dis- 


8          LESSON  XXIII  —  EQUILIBRIUM   DIAGRAM  OF  IRON-CARBON  ALLOYS 

tribution  of  graphitic  carbon  to  be  observed  in  the  case  of  gray  cast-iron  castings.  On 
the  contrary,  on  the  assumption  that  graphite  results  from  the  breaking  up  of  cemen- 
tite  soon  after  its  solidification,  it  is  readily  understood  why,  in  spite  of  their  very 
great  difference  in  specific  gravity,  iron  and  graphite  are  found  uniformly  distributed 
in  the  various  parts  of  castings.  The  microscopical  examination  of  the  structure 
of  very  slowly  cooled  castings  does  not  reveal  the  existence  of  a  graphite-austenite 
eutectic. 

That  cementite  is  unstable,  being  readily  converted  into  iron  and  graphitic  carbon, 
is  also  generally  admitted.  It  is  upon  this  instability  of  cementite  that  the  important 
industrial  operation  of  converting  white  cast-iron  castings  into  graphitic  malleable 
castings  is  based.  And  there  is  abundant  evidence  that  the  higher  the  temperature, 
the  more  readily  is  cementite  dissociated,  from  which  it  follows  that  the  higher  the 
temperature  at  which  cementite  forms  the  more  readily  will  it  be  converted  into 
graphitic  carbon.  Bearing  this  in  mind,  and  with  the  assistance  of  the  diagram  of 
Figure  3,  let  us  look  more  closely  into  the  graphitizing  of  cementite.  The  diagram 
shows  clearly  that,  during  the  solidification  of  alloys  containing  more  than  1.70  per 
cent  of  carbon,  (1)  some  cementite  forms  as  pro-eutectic  cementite  if  the  metal  con- 
tains more  than  4.3  per  cent  carbon,  (2)  some  cementite  forms  as  eutectic  cementite  in 
all  alloys,  (3)  some  cementite  remains  dissolved  in  the  eutectic-austenite  of  all  alloys, 
and  (4)  some  cementite  remains  dissolved  in  the  free  austenite  of  alloys  containing  less 
than  4.3  per  cent  carbon.  Considering  first  the  free  cementite,  that  is,  the  pro-eutectic 
and  the  eutectic  cementite,  it  is  evident  that  the  former  is  formed  at  a  higher  tempera- 
ture, and  that  the  more  carbon  in  the  alloy  the  higher  the  temperature  at  which  it 
begins  to  form.  It  seems  safe  to  infer,  therefore,  that  pro-eutectic  cementite  will  break 
up  into  graphite  and  ferrite  more  readily  than  eutectic  cementite,  this  being  consistent 
with  the  well-known  fact  that  hyper-eutectic  alloys  are  generally  rich  in  graphite  even 
after  relatively  quick  cooling.  The  presence  of  pro-eutectic  cementite  may  also  pro- 
mote the  formation  of  graphitic  carbon  because  once  this  graphitizing  process  is 
started,  it  is  likely  to  extend,  if  time  be  given,  to  the  bulk  of  the  cementite,  first  the  eu- 
tectic cementite  and  later  the  austenite-cementite  undergoing  the  change.  Alloys  con- 
taining less  than  4.3  per  cent  carbon  and  consequently  free  from  pro-eutectic  cementite 
should  not  become  graphitic  as  readily  because  of  the  lower  temperature  at  which 
eutectic-cementite  forms.  If  a  large  proportion  of  cementite  be  formed,  however, 
that  is,  if  the  alloys  contain  more  than  3  or  3.5  per  cent  of  carbon,  a  certain  amount 
of  graphitizing  is  readily  induced  through  slow  cooling.  With  decreasing  carbon 
the  breaking  up  of  cementite  becomes  progressively  more  difficult  until,  in  alloys  con- 
taining less  than  1.7  per  cent  carbon  (the  steel  series),  and,  therefore  free  from  eutectic 
cementite,  graphitic  carbon  is  very  seldom  formed. 

It  should  be  borne  in  mind  that  while  those  alloys  which  contain  but  a  small  pro- 

)rtioi.  oi^carbon  cannot  be  made  graphitic,  when  a  large  proportion  of  carbon  is 

'he  graphitizing  once  started  may  be  made  to  include  the  totality  of  the 

'iiib^iue,  thus  explaining  the  freedom  of  steel  from  graphite  and  the  freedom  of 

some  cast  irons  from  cementite. 

The  foregoing  remarks  apply  to  pure  iron-carbon  alloys,  the  influence  of  the 
elements  generally  present  in  commercial  products  having  been  purposely  ignored. 
When  dealing  with  commercial  steel  and  cast  iron,  the  well-known  influence  of  silicon 
in  promoting  the  formation  of  graphitic  carbon  should  be  remembered  as  well  as  the 
opposite  influence  of  sulphur  and  manganese.  Because  of  the  presence  of  a  notable 


f£  ,     . 

*1S|'    >  X 

.-.  w^^ffifciv''.         ^    - 


Fig.  7.  —  Magnified  750  diameters. 


Fig.  6.  —  Magnified  50  diameters. 


Fig.  9. —  Magnified  750  diameters. 


Fig.  8.  —  Magnified  50  diameters 


Fig.  10.  —  Magnified  50  diameters.  Fig.  11.  —  Magnified  750  diameters. 

Figs,  ft  and  7.  —  Iron-carbon  alloy.  Hypo-cutcctic.  Structure  immediately  after  solidification.  Dark  crystallites  of 
saturated  austenitc  in  a  matrix  of  austenite-cementitc  eutectic.  Figs.  8  and  9.  —  Iron-carbon  alloy.  Austenite- 
cementite  outrctic.  Irigs.  10  and  11.  —  Iron-carbon  alloy.  Hyper-eutectic.  Structure  immediately  after  solidifi- 
cation. Needles  of  cementite  in  a  matrix  of  austenite-cemcntite  eutectic.  (Gcerens.) 

9 


10       LESSON  XXIII  —  EQUILIBRIUM   DIAGRAM   OF  IRON-CARBON  ALLOYS 

proportion  of  silicon,  commercial  cast  irons  after  slow  cooling  are  necessarily  more 
graphitic  than  pure  alloys  of  same  carbon  content. 

Structure  of  Iron-Carbon  Alloys  Immediately  after  Solidification.  —  If  the  alloy 
contains  less  than  some  1.70  per  cent  carbon  it  is  made  up  after  complete  solidification 
of  crystalline  grains  of  austenite.  It  has  been  explained,  however,  that  in  the  absence 
of  manganese  or  other  "retarding"  elements  it  is  not  possible  to  prevent,  even  through 
very  rapid  cooling,  the  transformation  of  some  of  the  austenite  at  least  into  martensite. 
The  polyhedric  structure  of  austenite  has  been  illustrated  in  these  lessons  in  the  case 
of  special  steels  (manganese  and  nickel  steels)  and  it  is  now  well  understood  that  the 


ISOO 


6.67 


Fig.  12.  —  Equilibrium  diagram  of  iron-carbon  alloys. 


frequent  network  structures  of  slowly  cooled  steel  are  due  to  the  existence  of  poly- 
hedric austenitic  structures  above  their  critical  range. 

If  the  alloy  contains  from  1.70  to  4.3  per  cent  carbon  it  is  made  up,  after  solidifi- 
cation, of  crystals  of  saturated  austenite  and  of  eutectic  alloy.  This  is  well  shown 
after  Gcerens  in  Figure  6,  in  which  the  dark  "pine  tree"  crystals  consist  of  saturated 
austenite,  while  the  ground  mass  is  the  cementite-austenite  eutectic.  In  Figure  7  the 
same  structure  is  shown  more  highly  magnified.  If  the  alloy  contains  exactly  4.3  per 
cent  carbon,  it  consists  wholly  of  eutectic  as  shown  under  different  magnifications  in 
Figures  8  and  9.  It  has  been  seen  that,  theoretically,  this  eutectic  contains  47.7  per 
cent  of  saturated  austenite  (the  dark  constituent),  and  52.3  per  cent  of  cementite. 
Alloys  containing  more  than  4.3  per  cent  carbon  consist  after  solidification  of  free 
cementite  in  the  form  of  needles  embedded  in  a  eutectic  matrix  as  shown  in  Figures  10 
and  11. 

It  should  be  noted  that  the  dark  constituent  occurring  in  these  structures  and 
described  as  saturated  austenite  may  not  be  absolutely  unaltered  austenite  because  of 


LES30X   XXIII  — EQUILIBRIUM   DIAGRAM   OF   IROX-CARBOX   ALLOYS         11 


00 

S0* 

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Ul£ 


12       LESSON  XXIII  —  EQUILIBRIUM   DIAGRAM   OF  IRON-CARBON  ALLOYS 

the  difficulty  of  completely  preventing  the  transformation  of  that  constituent  even 
in  the  presence  of  a  large  amount  of  carbon  and  by  very  sudden  cooling.  If  the  aus- 
tenite  has  undergone  some  transformation,  however,  so  that  it  contains  some  mar- 
tensite  and  even  troostite  those  transformations  must  have  taken  place  in  situ  and  the 
structures  reproduced  in  Figures  6  to  11  must  represent  accurately  the  structural 
aspect  of  the  corresponding  alloys  after  solidification. 

Complete  Equilibrium  Diagram.  —  In  the  foregoing  pages  only  the  solidification 
curves  of  iron-carbon  alloys  have  been  considered  and  the  probable  mechanism  of 
their  freezing  explained.  Their  equilibrium  diagram,  however,  must  include  all  heat 
evolutions  observed  on  cooling  from  the  liquid  condition  to  atmospheric  temperature ; 
in  other  words,  the  thermal  critical  points  fully  described  in  previous  lessons  are  part 
of  the  complete  equilibrium  diagram  as  indicated  in  Figure  12. 

Since  the  meaning  of  every  curve  of  this  diagram  has  been  discussed,  it  only  re- 
mains to  inquire  into  any  possible  structural  or  other  changes  taking  place  after- 
solidification  and  before  the  alloys  reach  their  respective  thermal  critical  point  or 
points,  that  is,  while  they  cool  from  the  solidus  LSS'  to  the  eutectoid  line  CDF .  The 
changes  which  do  or  may  take  place  as  the  alloys  cool  in  this  range  are  clearly  stated 
in  Figure  13.  In  this  diagram  the  most  likely  significance  of  every  curve  is  indicated 
as  well  as  the  nature  of  all  structural  transformations,  and  of  all  possible  resulting 
structures  after  complete  cooling.  The  author  believes  that  it  embodies  those  infer- 
ences best  supported  by  analogy  and  by  experimental  evidences.  Although  neces- 
sarily involving  some  repetition,  a  methodical  examination  of  the  various  parts  of 
this  complete  diagram  seems  advisable,  as  it  will  permit  a  recapitulation  of  the  various 
matters  previously  discussed. 

Let  us  consider  (1)  the  solidification  of  iron-carbon  alloys,  (2)  their  cooling  from 
the  solidus  LSES'  to  the  eutectoid  temperature  CDF,  and  (3)  their  cooling  through 
the  eutectoid  temperature  and  their  final  structures. 

According  to  the  mechanism  of  their  solidification  iron-carbon  alloys  are  divided 
into  three  classes,  namely,  (1)  alloys  containing  less  than  1.70  per  cent  of  carbon,  (2) 
alloys  containing  between  1.70  and  4.3  per  cent  carbon,  and  (3)  alloys  containing 
more  than  4.3  per  cent  of  carbon.  Alloys  containing  less  than  1.70  per  cent  of  carbon 
and  including,  therefore,  all  the  steels  of  commerce  solidify  as  solid  solutions  of  the 
carbide  Fe3C  (cementite)  in  gamma  iron,  these  solutions  being  known  as  austenite. 
LA  is  the  liquidus  and  LS  the  solidus  of  these  alloys.  Alloys  containing  between  1.70 
and  4.3  per  cent  of  carbon  solidify  through  the  formation  of  crystals  of  saturated 
austenite  at  gradually  decreasing  temperatures  and  through  the  final  solidification, 
at  the  eutectic  temperature,  of  the  residual  molten  metal  necessarily  of  eutectic  com- 
position. Alloys  containing  more  than  4.3  per  cent  of  carbon  solidify  through  the 
formation  of  cementite  crystals  at  gradually  decreasing  temperatures,  and  through  to 
final  solidification,  at  the  eutectic  temperature,  of  the  residual  molten  metal  necessarily 
of  eutectic  composition. 

In  cooling  below  their  solidus,  LS,  alloys  with  less  than  1.70  per  cent  carbon  undergo 
no  change  until  they  reach  their  thermal  critical  points  Ar3,  Ar3.2,  Ar3.2.i  or  Arcm  as 
the  case  may  be,  when,  if  they  contain  less  than  some  0.85  per  cent  of  carbon  (hypo- 
eutectoid  steels),  some  iron  is  set  free  and  converted  into  beta  iron,  while  if  they 
contain  more  carbon  (hyper-eutectoid  steels),  cementite  is  liberated.  In  either  case 
when  the  eutectoid  temperature  is  reached  the  residual  austenite,  now  of  eutectoid 
composition  (0.85  per  cent  carbon),  is  converted  into  pearlite. 


LESSON   XXIII  —  EQUILIBRIUM    DIAGRAM   OF   IRON-CARBON   ALLOYS        13 

iSOO. 2 ^ .  .  6 


Pro-eufecf/c 
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Iron 
d Hut  in 
saturate 
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•Struct  u  rot 
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Co m po*s  i  fton 
i mm  edict te/y 
above 
Ar, 


Structural 
compos  it  ion 


6          6.57 
Carhon  °/o        » 

Fig.  14.  —  Equilibrium  and  structural  composition  diagram  of  iron-carbon  alloys. 


14        LESSON    XXIII  —  EQUILIBRIUM    DIAGRAM  OF    IRON-CARBON   ALLOYS 

After  solidification,  alloys  containing  between  1.70  and  4.3  per  cent  carbon  are 
aggregates  of  saturated  austenite  (austcnite  containing  1.70  per  cent  C  or  25.5  per 
cent  cementite)  and  of  cementite-austenite  eutectic.  On  cooling  below  their  solidus, 
cementite  (pro-eutectoid  cementite)  is  liberated  both  from  the  free  and  from  the 
eutectic-austenite,  while  if  the  cooling  be  sufficiently  slow  both  the  eutectic  and  pro- 
eutectoid  cementite  may  be  partly  or  wholly  dissociated  into  graphite  and  iron  (f errite) . 
Indeed,  the  graphitizing  may  even  include  the  eutectoid  cementite,  in  which  case  the 
alloy  is  made  up  solely  of  ferrite  and  graphite. 

Alloys  containing  more  than  4.3  per  cent  of  carbon  are,  immediately  after  solidi- 
fication, aggregates  of  cementite  (pro-eutectic  cementite)  and  of  cementite-austenite 
eutectic.  On  slow  cooling  below  their  solidus,  cementite  (pro-eutectoid  cementite)  is 
liberated  from  the  eutectic  austenite  while  the  pro-eutectic,  eutectic,  and  pro-eutectoid 


Fig.  15.  —  The  author's  early  equilibrium  diagram  (1896). 

cementite  may  be  partly  or  wholly  dissociated  into  graphite  and  ferrite,  in  some  ex- 
treme cases  the  eutectoid  cementite  even  being  graphitized. 

On  cooling  through  the  eutectoid  temperature,  any  remaining  austenite  is  bodily 
converted  into  pearlite. 

The  above  consideration  clearly  shows  that  in  alloys  containing  more  than  some 
1.70  per  cent  carbon  four  types  of  structure  may  be  formed  according  to  the  rate  of 
cooling  below  the  solidus: 

(I)  Cementite  plus  pearlite,  the  structure  of  white  cast  iron,  readily  produced  by 
quick  cooling  and  representing  a  metastable  equilibrium.  (II)  Ferrite  and  graphite, 
an  extreme  case,  possible  only  after  very  slow  cooling  and  in  the  presence  of  much 
silicon,  little  manganese,  and  sulphur,  and  representing  stable  equilibrium.  (Ill)  Cem- 
entite plus  pearlite  and  graphite,  the  structure  of  gray  cast  iron  with  hyper-eutectoid 
matrix,  resulting  from  slow  cooling,  promoted  by  the  presence  of  silicon  and  opposed 
by  sulphur  and  manganese.  (IV)  Pearlite,  ferrite,  and  graphite,  the  structure  of  gray 
cast  iron  with  hypo-eutectoid  matrix,  produced  because  of  slower  cooling  or  because 
of  the  presence  of  more  silicon  or  of  less  sulphur  and  manganese. 


LESSON  XXIII  —  EQUILIBRIUM   DIAGRAM   OF  IRON-CARBON  ALLOYS        15 

It  will  be  explained  in  the  next  lesson  that,  according  to  the  phase  rule,  only  two 
components  may  be  present  in  a  binary  alloy  in  a  state  of  equilibrium  from  which 
it  follows  that  gray  cast  irons,  since  they  contain  besides  iron  both  cementite  and 
graphite,  are  out  of  equilibrium.  One  of  the  components  must  disappear  if  the  alloy 
is  to  assume  equilibrium.  Cementite  is  undoubtedly  that  component  as  proven  by 
the  malleablizing  of  cast  iron  when  cementite  is  readily  dissociated  into  graphite  and 
ferrite  on  prolonged  heating  to  a  high  temperature. 

In  Figure  14  the  complete  equilibrium  diagram  is  shown  combined  with  three 
constitutional  diagrams  showing  graphically  the  structural  composition  of  iron-car- 
bon alloys  (1)  immediately  after  their  solidification,  (2)  immediately  before  the  eutec- 
toid  temperature,  and  (3)  below  the  eutectoid  temperature,  ^nlhe  assumption  that 
no  graphitic  carbon  is  formed.  The  structural  changes  taking  place  while  the  alloys 


1.600- 
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Fig.  16.  —  Roberts-Austen's  first  equilibrium  diagram  (1897). 


cool  below  their  solidus  down  to  the  eutectoid  temperature  are,  in  this  way,  clearly 
depicted.  The  following  facts,  for  instance,  are  graphically  shown,  (1)  the  pro-eutectic 
cementite  formed  during  the  solidification  of  hyper-eutectic  alloys  and  the  eutectic 
cementite  present  in  all  alloys  containing  more  than  1.70  per  cent  carbon  remain  un- 
changed as  the  alloy  cools  to  atmospheric  temperature,  (2)  the  free  saturated  austenite 
of  hypo-eutectic  alloys,  as  well  as  the  eutectic-austenite,  are  converted  into  eutectoid 
austenite  through  the  liberation  of  cementite  (pro-eutectoid  cementite),  the  area 
EDH  representing  the  cementite  thus  set  free,  and  (3)  in  hypo-eutectoid  alloys  iron 
is  set  free  as  shown  by  the  area  FJG.  The  lower  diagram  shows  that,  in  cooling 
through  the  eutectoid  temperature,  the  remaining  austenite,  now  necessarily  of" 
eutectoid  composition,  and  sometimes  called  hardenite  is  converted  into  pearlite. 
Taking,  for  instance,  the  metal  whose  composition  and  temperature  are  represented 
by  the  point  R,  its  transformations  and  final  structure  are  clearly  shown.  At  M  it 
begins  to  solidify  through  the  formation  of  crystals  of  saturated  austenite;  from  M  to 
N  the  austenite  crystals  continue  to  grow,  the  percentage  of  free  austenite  present  in 
the  solidified  metal  being  represented  by  the  distance  OP;  at  N  the  residual  bath 


16        LESSON   XXIII  —  EQUILIBRIUM    DIAGRAM   OF   IRON-CARBON   ALLOYS 

solidifies  as  a  eutectic  alloy,  the  percentage  of  which  is  proportional  to  the  distance 
KO;  this  eutectic  contains  KL  =  PQ  =  TU  per  cent  of  cemeiitite  and  LO  per  cent 
of  saturated  austenite;  LP  is  the  total  amount  of  austenite  in  the  alloy;  after  solidifi- 
cation in  cooling  from  N  to  K,  pro-eutectoid  cementite  is  liberated  both  from  the 
free  and  from  the  eutectic-austenite,  QS  representing  the  percentage  of  cementite 
finally  expelled;  on  reaching  the  point  K,  the  remaining  austenite,  ST,  is  of  eutectoid 
composition,  when  it  is  sometimes  called  hardenite,  and  in  cooling  through  K  this 
austenite  is  converted  into  pearlite,  the  metal  being  finally  made  up  of  TU  per  cent 
of  eutectic  cementite,  UV  per  cent  of  pro-eutectoid  cementite,  and  VX  per  cent  of 
pearlite,  the  latter  containing  VW  per  cent  of  cementite,  and  WX  per  cent  of  ferrite, 
or  of  TV  per  cent  of  free  cementite  and  VX  per  cent  of  pearlite,  or  again  of  TW 
per  cent  of  total  cementite  and  WX  per  cent  of  ferrite. 

Historical.  —  In  view  of  the  scientific  and  practical  importance  of  the  equilibrium 


VSOO 


2-S    If    3-0     3-2    3-f    3-S 

CARBON  PER  CCNT 


5-0     SI    54      *6 


Fig.  17.  —  Roberts-Austen's  second  equilibrium  diagram  (1899). 

diagram  of  iron-carbon  alloys,  a  brief  historical  sketch  of  its  evolution  should  be  of 
interest  to  the  reader.  The  first  diagram  was  published  by  the  author  in  1896. '  It  is 
reproduced  in  Figure  15.  It  will  be  noted  that,  although  the  diagram  includes  only 
the  thermal  critical  points,  it  is  otherwise  substantially  accurate.  In  describing  it 
the  author  wrote  in  part : 

"Figure  1  shows  graphically  the  position  of  the  critical  points  in  cooling  steels  of 
various  carbons.  The  width  of  the  black  lines  does  not  refer  to  the  intensity  of  the 
retardations,  but  only  indicates  the  range  of  temperature  which  they  cover.  For 
instance,  it  shows  that  the  single  retardation  of  high  carbon  steel  begins  at  about 
680  deg.  C.  and  ends  at  about  640  deg.  C.  The  maximum  evolution  of  heat  lies  some- 
where between  these  limits,  but  not  necessarily  in  the  middle. 

"  This  graphical  representation  was  obtained  by  plotting  the  results  of  the  investi- 
gations of  Osmond,  Howe,  Roberts- Austen,  Arnold,  and  the  writer;  and,  with  one  or 
two  exceptions  all  their  figures  fall  very  nearly  within  the  limits  here  indicated." 

1  "The  Microstructure  of  Steel  and  the  Current  Theories  of  Hardening,"  ALBERT  SAUVEUK, 
Transactions  American  Institute  of  Mining  Engineers.  1896,  p.  867. 


LESSON   XXIII  —  EQUILIBRIUM   DIAGRAM   OF   IRON-CARBON  ALLOYS        17 

It  is  from  this  modest  beginning  that  the  present  diagram  was  evolved. 

In  1897  Roberts-Austen  published  in  his  fourth  report  of  the  Alloys  Research  Com- 
mittee of  the  Institution  of  Mechanical  Engineers  the  diagram  reproduced  in  Figure  16. 

Two  years  later,  in  1899,  the  diagram  shown  in  Figure  17  was  published  by  Roberts- 
Austen  and  Stansfield  in  the  fifth  report  of  the  Alloys  Research  Committee.  Some  of 
the  conspicuous  features  of  this  diagram  should  be  noted.  The  solidification  point  of 
pure  iron  was  indicated  to  be  1600  degrees  C.  whereas  we  know  now  that  it  is  nearly 
1500  deg.  No  attempt  had  been  made  yet  at  ascertaining  the  end  of  the  solidification, 
that  is,  the  solidus,  of  alloys  forming  solid  solutions;  the  formation  of  a  eutectic  on 
solidification  was  indicated  as  taking  place  in  alloys  containing  more  than  one  per  cent 
carbon;  graphite  was  supposed  to  crystallize  during  the  solidification  of  alloys  con- 


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Fig.  18.  —  Roozeboom's  equilibrium  diagram  (1900). 

taining  more  than  4.3  per  cent  carbon,  there  being  in  the  diagram  no  indications  of 
possible  formation  of  cementite;  the  eutectic  alloy  was  assumed  to  be  a  graphite-iron 
eutectic;  critical  points  occurring  below  the  eutectoid  temperature  were  represented 
in  the  diagram  and  marked  "hydrogen  points"  (See  Lesson  VII,  page  8,  "Minor 
critical  points");  the  Arcm  curve  was  arbitrarily  extended  to  yield  a  V-shaped  curve. 
Roberts-Austen  mentioned  the  formation  of  a  solid  solution,  free  in  hypo-eutectic 
steels,  and  as  a  constituent  of  the  eutectic  in  alloys  of  eutectic  composition,  and  he 
ascribed  the  presence  of  free  cementite  in  cast  iron  to  the  liberation  of  that  constituent 
from  solid  solution. 

In  1900  Roozeboom  took  up  the  study  of  Roberts-Austen's  diagram,  and  applying 
to  it  the  teachings  of  the  phase  rule  published  the  diagram  of  Figure  18  as  a  probably 
accurate  representation  of  the  solidification  mechanism  of  iron-carbon  alloys  and  of 
the  structural  transformations  taking  place  after  solidification.  In  this  diagram,  the 
line  ba,  that  is,  the  solidus  of  alloys  forming  solid  solutions,  is  for  the  first  time  indicated; 


18        LESSON  XXIII  —  EQUILIBRIUM   DIAGRAM  OF  IRON-CARBON  ALLOYS 


1500 


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Fig.  19.  —  Carpenter's  and  Keeling's  equilibrium  diagram  (1904). 


LESSON  XXIII  —  EQUILIBRIUM   DIAGRAM   OF   IRON-CARBON  ALLOYS         19 


the  word  martensite  is  used  instead  of  austenite  to  denote  the  solid  solution  of  iron 
and  carbon;  free  graphite  is  assumed  to  form  during  the  solidification  of  alloys  con- 
taining more  than  4.3  per  cent  carbon  and  the  constituents  of  the  eutectic  alloy  are 
supposed  to  be  martensite  (solid  solution)  and  graphite.  The  Arcm  curve  of  hyper- 
eutectoid  steels  is  arbitrarily  extended  as  an  horizontal  line  starting  from  E  (1.75  per 
cent  carbon  and  1000  deg.  C.)  and  extending  to  the  end  of  the  diagram.  Roozeboom 
argued  that,  while  graphite  formed  on  solidification  in  all  alloys  containing  more  than 
2  per  cent  carbon,  this  graphite  at  1000  deg.  (line  EF)  recombined  with  iron  to  yield 
cementite  so  that,  finally,  alloys  in  equilibrium  would  contain  only  ferrite  and  cementite, 
thus  conforming  to  the  phase  rule  which  forbids  the  presence  of  more  than  two  com- 
ponents in  binary  alloys.  It  has  since  been  conclusively  shown  that  Roozeboom  was 
in  error,  that  while  the  ferrite-cementite  system  is  in  equilibrium  according  to  the 

TEMPER- 
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Fig.  20.  —  Benedicks'  double  equilibrium  diagram. 

phase  rule,  it  is  in  metastable  equilibrium,  the  ferrite-graphite  system  being  the  only 
stable  one.  The  hypothetical  horizontal  line  EF  is  now  consequently  omitted  from 
the  equilibrium  diagram,  and  the  Arcm  curve  made  to  join  the  eutectic  line  at  its 
origin  (a). 

In  1904  Carpenter  and  Keeling  made  a  series  of  very  careful  experiments  in  order 
to  ascertain  the  evolutions  of  heat  taking  place  in  cooling  pure  iron-carbon  alloys  from 
the  liquid  state  to  atmospheric  temperature.  By  plotting  their  results,  the  equilibrium 
diagram  reproduced  in  Figure  19  was  obtained.  The  solidification  of  pure  iron  is 
shown  to  take  place  at  1500  deg.  C.  The  curves  are  otherwise  identical  to  those  of 
Roozeboom,  the  horizontal  line  EF  having  been  introduced.  The  faint  evolutions  of 
heat  occurring  in  the  vicinity  of  600  deg.  C.  already  discovered  by  Roberts-Austen 
and  ascribed  by  him  to  the  presence  of  hydrogen,  were  also  observed  by  Carpenter 
and  Keeling,  as  well  as  some  faint  evolutions  in  the  vicinity  of  775  deg.,  the  meaning 
of  which  remains  uncertain. 


20       LESSON  XXIII  —  EQUILIBRIUM   DIAGRAM  OF  IRON-CARBOX  ALLOYS 


When  it  became  apparent  that  graphite  and  not  cementite  must  be  the  final  stable 
form  of  carbon,  several  authorities  argued  that  two  equilibrium  conditions  could  exist 
according  to  the  rate  of  cooling  during  solidification,  one  of  them  stable,  the  other 
metastable,  and  that  this  should  be  indicated  in  the  diagram.  This  view  was  presented 
notably  by  Charpy  and  Grenet,  by  Benedicks  and  by  Heyn.  The  double  diagram 
advocated  by  them  is  represented  in  Figure  20.  The  solidification  of  free  cementite 
and  of  the  cementite-austenite  eutectic  being  assumed  to  be  due  to  the  well-known 
phenomenon  of  surfusion  or  undercooling,  the  corresponding  curves  are  arbitrarily 
outlined  at  temperatures  slightly  lower  than  those  pertaining  to  the  formation  of  free 


500' 


Cent,. 


Fig.  21. 


Carbon,   per 
•  Rosenhain's  equilibrium  diagram  (1911). 


graphite  and  of  graphite-austenite  eutectic.  The  author  has  already  shown  why,  in 
his  opinion,  the  graphite  curves  should  be  left  out.  The  view  that  cementite  always 
forms  during  the  solidification  of  iron-carbon  alloys  but  that  being  unstable  it  is 
readily  dissociated  into  ferrite  and  graphite,  seems  to  be  better  supported  by  experi- 
mental evidences  and  more  consistent  with  practical  facts. 

Rosenhain  has  recently  plotted  the  experimental  results  of  Carpenter  and  Keel- 
ing, of  Gutowsky  and  of  himself,  obtaining  the  diagram  reproduced  in  Figure  21.  He 
considers  Gutowsky's  results  in  regard  to  the  form  of  the  solidus  curve  of  alloys  form- 
ing solid  solutions  as  more  accurate  than  those  previously  published,  and  he  incorpo- 
rates them  in  the  diagram  as  shown  in  Figure  21,  the  solidus  line  being  rounded  instead 
of  straight  as  heretofore  represented.  In  justification  of  his  course,  Rosenhain  writes: 

"We  have  now  to  consider  the  curved  portion  of  the  'solidus,'  the  line  AD.    This 


LESSON   XXIII  —  EQUILIBRIUM    DIAGRAM   OF   IRON-CARBON   ALLOYS        21 

represents  the  temperatures  at  which  the  alloys  have  just  completed  their  freezing 
process,  that  is,  have  just  become  completely  solid,  or,  conversely,  it  represents  the 
temperature  of  incipient  fusion  on  heating.  In  the  earlier  investigations,  and  even  in 
those  of  Messrs.  Carpenter  and  Keeling,  these  temperatures  were  obtained  by  esti- 
mating the  point  on  each  of  the  cooling-curves  where  the  heat-evolution  due  to  solidi- 
fication came  to  an  end.  Unfortunately,  the  end  of  a  heat-evolution  is  never  sharply 
indicated  on  the  curves,  so  that  this  estimation  was  admittedly  vague.  Quite  recently 
that  determination  has  been  repeated,  and  with  considerable  greater  accuracy,  be- 
cause a  very  much  more  satisfactory  method  was  available  .  .  . 

"The  method  of  determining  the  'solidus'  was  to  take  small  pieces  of  steel,  of 
known  composition,  heat  them,  and  suddenly  cool  them  from  successively  higher 
temperatures;  afterward  each  specimen  was  examined  by  means  of  the  microscope. 


ig.  22. Cooling  curves  of  carbon  steels  replotted  from  the  data  of  Carpenter  and  Keeling.    (Rosenhain.) 

It  is  easy,  as  the  photographs  show,  to  determine  what  is  the  particular  point  at  which 
you  have  reached  a  temperature  where  there  was  a  small  quantity  of  liquid  metal 
present  at  the  moment  of  quenching." 

The  data  obtained  by  Carpenter  and  Keeling  have  been  given  in  the  form  of  a  table 
in  Lesson  VII,  page  9,  and  some  of  these  curves  reproduced  in  Figure  8,  page  17  of  the 
same  lesson.  Rosenhain  has  recently  replotted  some  of  the  figures  of  Carpenter  and 
Keeling  by  his  derived  differential  method  (Lesson  VII,  page  16)  and  obtained  the 
sharp  curves  shown  in  Figure  22. 

Examination 

Describe  briefly  (1)  the  solidification,  (2)  the  transformations  after  solidification 
and  according  to  rate  of  cooling,  and  (3)  the  final  structures  of  iron-carbon  alloys  con- 
taining respectively  1.25,  3.50,  and  5.00  per  cent  carbon. 

Calculate  the  structural  composition  of  these  alloys,  assuming  that  graphitic 
carbon  does  not  form,  (1)  immediately  after  solidification,  and  (2)  below  the  eutectoid 
temperature. 


LESSON  XXIV 

THE  PHASE  RULE 

The  Phase  Rule  to  which  references  have  been  made  in  the  preceding  lessons 
should  now  be  considered  as  it  has  been  found  of  much  assistance  in  interpreting  cor- 
rectly the  iron-carbon  equilibrium  diagram. 

Enunciation  of  the  Phase  Rule.  —  The  phase  rule  was  enunciated  in  1878  by  Wil- 
lard  Gibbs,  at  the  time  Professor  of  Physics  in  Yale  University.  It  is  one  of  the  most 
notable  contributions  ever  made  to  physical  chemistry. 

The  phase  rule  deals  with  the  equilibrium  of  systems  and  is  generally  expressed 

by  the  formula : 

F  =  C  +  2-P 

showing  the  relation  existing  between  the  degrees  of  freedom  (F)  of  a  system,  the  num- 
ber of  components  (C),  and  the  number  of  phases  (P);  it  tells  us  that  the  number  of 
degrees  of  freedom  of  any  system  is  equal  to  the  number  of  its  components  plus  two, 
minus  the  number  of  phases  present.  In  order  to  understand  the  phase  rule  and  its 
application,  it  is  necessary  and  sufficient  to  have  an  accurate  understanding  of  the 
meaning  of  the  terms  employed  in  its  enunciation,  namely,  equilibrium,  degrees  of 
freedom,  components,  and  phases. 

Equilibrium.  —  A  substance  or  system  may  be  said  to  be  in  a  state  of  equilibrium 
when  it  is  chemically  and  physically  at  rest,  meaning  by  chemical  rest  that  chemical 
compounds  are  neither  being  tlissociated  nor  formed,  and  by  physical  rest,  not  the 
absence  of  motion  but  the  absence  of  molecular  transformation,  such  as  changes  of 
state  or  allotropic  changes.  It  is  necessary,  however,  to  distinguish  between  homo- 
geneous and  heterogeneous  substances.  A  substance  is  said  to  be  homogeneous  when  it 
is  chemically  and  physically  uniform  throughout,  i.e.  when  any  two  portions  of  it  pos- 
sess identical  chemical  and  physical  properties.  Homogeneous  substances  are  neces- 
sarily gaseous  mixtures,  elements,  chemical  compounds,  or  liquid  and  solid  solutions. 
The  equilibrium  of  a  homogeneous  system  is  sometimes  called  homogeneous  equi- 
librium. A  heterogeneous  substance  is  made  up  of  two  or  more  physically  separate 
parts,  that  is,  of  parts  having  different  physical  properties.  Ice  and  water,  many 
rocks,  and  many  alloys  are  instances  of  heterogeneous  substances.  If  the  com- 
ponent parts  of  heterogeneous  substances  may  be  present  in  indefinite  proportions, 
the  substances  are  mechanical  mixtures;  if  they  occur  in  definite  proportions,  the 
substances  are  eutectic  or  eutectoid  alloys.  The  equilibrium  of  heterogeneous  systems 
is  sometimes  called  heterogeneous  equilibrium. 

Howe  has  recently  suggested  that  the  homogeneous  constituents  of  alloys  be  called 
"metarals"  because  of  the  great  analogy  between  the  constitution  of  metallic  alloys 
and  of  rocks,  the  minerals  being  the  homogeneous  components  of  the  latter,  while  the 
word  aggregate  is  very  frequently  used  to  designate  heterogeneous  alloys.  In  metal- 

1 


2  LESSON   XXIV  — THE    PHASE    RULE 

lography,  therefore,  metarals  and  aggregates  may  conveniently  replace  the  equivalent 
terms,  homogeneous  and  heterogeneous  substances,  of  the  physical  chemist. 

Only  three  independently  variable  factors  can  affect  the  equilibrium  of  a  system, 
namely,  (1)  the  temperature,  (2)  the  pressure,  and  (3)  the  concentration  or  composi- 
tion. If  arbitrary  values  may  be  given  to  one  or  more  of  these  factors  without  destroy- 
ing the  chemical  and  physical  rest  of  the  system,  its  equilibrium  is  said  to  be  stable. 
On  the  contrary,  if  a  change  in  value  of  any  one  of  these  three  factors  results  in  chemi- 
cal or  physical  transformation,  i.e.  in  atomic  or  molecular  activity  such  as  dissociation 
or  formation  of  chemical  compounds,  changes  of  state,  or  allotropic  changes,  the 
equilibrium  of  the  system  was  unstable.  Water  under  atmospheric  pressure  is  in 
stable  equilibrium,  for  we  may  change  its  temperature  within  wide  limits  without 
causing  it  to  undergo  a  change  of  state,  while  of  course  its  chemical  composition 
remains  likewise  unaffected.  All  elements  are  generally  in  a  state  of  stable  equilib- 
rium, as  well  as  an  infinite  number  of  substances  composed  of  two  or  more  elements, 
for  they  may  be  heated,  for  instance,  through  wide  ranges  of  temperatures  without 
upsetting  their  physico-chemical  equilibrium.  Examples  of  unstable  equilibrium, 
however,  are  far  from  uncommon.  During  the  solidification  of  substances,  for  in- 
stance, stages  must  generally  be  passed  through  during  which  the  equilibrium  of  the 
substance  is  unstable,  and  it  is  often  possible  through  very  rapid  cooling  to  retain 
in  the  cold  the  unstable  conditions,  because  of  the  rigidity  of  the  substance  now 
opposing  the  changes  needed  for  a  return  to  stable  equilibrium.  It  has  been  seen 
in  these  lessons  that  hardened  steel  is,  for  the  above  reason,  unstable,  hence  the 
possibility  of  tempering  it  by  slight  reheating. 

The  kind  of  equilibrium  known  as  metastable  remains  to  be  described.  Liquids 
may  be  cooled,  when  taking  the  necessary  precautions,  below  their  normal  freezing- 
point,  without  freezing,  the  phenomenon  being  known  as  superfusion,  surfusion,  or 
undercooling,  and  the  substance  being  said  to  be  in  metastable  equilibrium. 
Water,  for  instance,  may  be  cooled  below  0  deg.  C.  and  still  remain  liquid. 
The  introduction  of  a  solid  fragment  of  the  substance,  a  piece  of  ice  in  the  case 
of  water,  results  in  the  solidification  of  the  liquid  while  its  temperature  rises  to  its 
normal  freezing-point.  Otherwise,  the  substance  may  be  kept  liquid  below  its 
solidification  point  for  any  length  of  time.  If  the  temperature  of  the  liquid  con- 
tinues to  fall,  however,  a  point  is  reached  when  its  .equilibrium  becomes  unstable,  i.e. 
when  further  lowering  of  temperature  causes  the  liquid  to  solidify.  To  state  the  case 
broadly,  the  failure  on  the  part  of  a  system  to  undergo  a  certain  chemical  or  physical 
transformation  when  that  transformation  is  due,  although  given  the  necessary  time, 
results  in  metastable  equilibrium,  while  its  failure  to  undergo  a  transformation  because 
of  the  necessary  time  being  denied,  as  in  quenching,  results  in  unstable  equilibrium. 
Metastable  equilibrium  is  stable,  at  least  within  narrow  limits  of  temperature,  while, 
theoretically  at  least,  slight  heating  of  a  substance  in  unstable  equilibrium  should 
result  in  a  partial  return  to  a  more  stable  condition,  that  is,  in  a  partial  occurrence  of 
the  transformation  that  was  suppressed  by  quick  cooling. 

Degrees  of  Freedom.  —  By  the  degrees  of  freedom  (sometimes  called  degrees  of 
liberty),  of  a  system  is  meant  the  number  of  the  three  independently  variable  factors, 
temperature,  pressure,  and  concentration,  which  may  arbitrarily  be  made  to  vary 
without  disturbing  the  system's  physico-chemical  rest.  It  has  already  been  noted  that 
a  system,  in  order  to  be  in  stable  equilibrium,  must  have  at  least  one  degree  of  free- 
dom. It  will  also  be  understood  that  no  system  can  have  more  than  two  degrees  of 
freedom  because  in  the  case  of  arbitrary  values  being  given  to  two  of  the  factors,  the 


LESSON   XXIV  — THE    PHASE   RULE  3 

value  of  the  third  is  necessarily  fixed,  this  being  due  to  the  known  rigid  relations 
existing  between  temperature,  pressure,  and  concentration. 

Systems  whioh  have  no  degree  of  freedom  are  said  to  be  "un variant"  or  ''non- 
variant."  Their  equilibrium  is  necessarily  unstable.  Systems  having  one  degree  of 
freedom  are  called  "univariant"  or  "mono variant,"  and  those  with  two  degrees  of 
freedom  "bivariant"  or  "divariant." 

Phases.  —  By  the  phases  of  a  system  are  meant  the  homogeneous,  physically  dis- 
tinguishable, and  mechanically  separable  constituents  of  that  system.  Water  and  ice, 
for  instance,  are  possible  phases  of  the  water-ice  system;  quartz,  felspar,  and  mica  are 
phases  of  quartz,  that  is,  of  the  silica-alumina-potash  system.  It  will  be  apparent  that 
phases  must  necessarily  be  gaseous  mixtures,  elements,  definite-chemical  compounds, 
or  solutions.  As  previously  mentioned,  Howe,  following  the  petrographical  nomencla- 
ture, and  noting  that  the  minerals  are  the  phases  of  rocks,  calls  "metarals"  the  phases 
of  metals  and  alloys. 

Components.  —  The  components  of  a  system  are  described  by  Findlay  as  "those 
constituents  the  concentration  of  which  can  undergo  independent  variation  in  the 
different  phases,"  by  Bancroft  as  "substances  of  independently  variable  concentra- 
tion," by  Mellor  as  those  "entities  which  are  undecomposable  under  the  conditions 
of  experiments,"  by  Howe  as  "free  elements  and  those  compounds  which  in  the  nature 
of  the  case  are  undecomposable  under  the  conditions  contemplated,  and  so  play  the 
part  of  elements."  The  components  of  a  system  may  be  either  chemical  compounds 
or  elements,  but  there  is  at  times  some  difficulty  in  grasping  the  distinction  between 
the  components  of  a  system  and  its  ultimate  chemical  constituents.  The  criterion  by 
which  to  decide  whether  an  entity  is  or  is  not  a  component,  is  the  possibility  of  in- 
dependent variation  in  the  different  phases.  Take  the  system  water,  for  instance: 
evidently  water  and  not  hydrogen  and  oxygen  is  the  component,  because  any 
variation  in  the  proportion  of  hydrogen  would  necessarily  imply  a  corresponding 
and  well-defined  variation  in  the  proportion  of  oxygen  and  vice  versa.  Findlay 
writes: 

"In  deciding  the  number  of  components  in  any  given  system,  not  only  must  the 
constituents  chosen  be  capable  of  independent  variation,  but  a  further  restriction  is 
imposed,  and  we  obtain  the  following  rule:  As  the  components  of  a  system  there  are  to  be 
chosen  the  smallest  number  of  independently  variable  constituents  by  means  of  which 
the  composition  of  each  phase  participating  in  the  state  of  equilibrium  can  be  expressed 
in  the  form  of  a  chemical  equation." 

In  the  case  of  alloys,  however,  such  difficulty  does  not  arise,  for  the  constituent 
metals  are  always  the  components  of  the  systems. 

The  Phase  Rule  Applied  to  Alloys.  —  In  dealing  with  alloys  we  may  for  all  practi- 
cal purposes  ignore  the  influence  of  pressure,  seeing  that  because  of  their  feeble  vola- 
tility they  are  practically  always  subjected  to  atmospheric  pressure.  Omitting  the 
influence  of  pressure  necessarily  reduces  by  one  the  possible  number  of  degrees  of 
freedom  so  that  in  the  case  of  alloys  the  phase  rule  may  be  expressed  by  the  formula : 

F=C+1-P 

signifying  that  the  number  of  degrees  of  freedom  is  equal  to  the  number  of  components 
plus  one,  minus  the  number  of  phases.  Since  to  be  in  stable  equilibrium  a  system  must 
have  at  least  one  degree  of  freedom,  it  is  obvious  that  an  alloy  made  up  of  n  metals 
cannot  have  more  than  n  phases.  If  it  had  n  +  1  phases  it  would  have  no  degree  of 
freedom,  that  is,  its  equilibrium  would  be  unstable.  With  n-  I  phases  it  would  have 


4  LESSON   XXIV  —  THE   PHASE   RULE 

two  degrees  of  freedom.    It  could  not  have  less  than  n  -  1  phases,  since  it  cannot  have 
more  than  two  degrees  of  freedom. 

The  Phase  Rule  Applied  to  Pure  Metals.  —  Pure  metals  have  only  one  component, 
hut  their  possible  phases  are  (1)  liquid  metal,  (2)  solid  metal,  (3)  several  allotropic  con- 
ditions of  the  solid  metal.  Let  us  consider  Figure  1,  which  represents  the  solidification 
of  a  pure  metal  as  explained  in  Lesson  XXII. 

Above  the  temperature  T  the  metal  is  entirely  liquid;  it  has  but  one  phase,  and 
consequently  one  degree  of  freedom  (F  =1  +  1-1  =  1).  The  system  above  T  is 
univariant;  its  temperature  may  be  altered  within  wide  limits  without  disturbing  its 


I 


F- /+/-/•=/ 
un/  vctr/anf 


F=/+/-2=o 
non-  var/an  t 


F~- /+/-/=/ 


/  / me 

Fig.   1.  —  Equilibrium  of  pure  metals  according  to  the  Phase  Rule. 

equilibrium:  it  remains  liquid.  At  the  temperature  T  two  phases  are  present,  solid 
metal  and  liquid  metal,  the  metal  having,  therefore,  no  degree  of  freedom  (F  =  1  +  1  - 
2  =  0):  it  is  non-variant.  Liquid  and  solid  metal  can  exist  only  at  one  temperature, 
the  critical  temperature  of  solidification,  any  change  of  its  temperature  resulting  in 
the  disappearance  of  one  of  the  phases,  that  is,  in  a  return  to  stable  equilibrium. 
Increasing  the  temperature  must  result  in  the  disappearance  of  the  solid  phase,  while 
lowering  the  temperature  must  cause  the  disappearance  of  the  liquid  phase.  Below 
the  temperature  T  the  system  contains  only  the  solid  phase,  being,  therefore,  univari- 
ant :  its  temperature  may  be  varied  arbitrarily. 

The  Phase  Rule  Applied  to  Binary  Alloys.  —  Binary  alloys  having  for  components 
the  two  alloying  metals,  the  formula  of  the  phase  rule  becomes: 

F  =  2+ 1 -P 
or  F  =  3  -  P 


LESSON   XXIV  — THE    PHASE    RULE  5 

Clearly  binary  alloys  when  in  a  condition  of  stable  equilibrium  cannot  have  more  than 
two  phases.  With  one  phase  they  will  be  bivariant,  with  two  phases  univariant,  and 
with  three  phases  non-variant.  Let  us  apply  the  rule  to  the  fusibility  curves  of  binary 
alloys  of  metals  partially  soluble  in  each  other  when  solid  (Fig.  2).  Above  the  liquidus 
MEM'  there  is  but  one  phase  present,  namely  the  liquid  phase,  the  system  being, 
therefore,  bivariant  (F  =  3  -  1  =  2),  i.e.  both  temperature  and  concentration  may  be 
varied  arbitrarily  without  upsetting  the  equilibrium  of  the  system,  which  means,  in 
the  case  under  consideration,  without  causing  its  solidification.  On  reaching  any 
point  L  of  the  liquidus  the  alloy  begins  to  solidify,  and  two  phases  are  now  present, 
namely,  solid  solution  and  liquid  alloy,  the  system  becomes  univariant  (F  =  3  -  2  =  1). 
Having  but  one  degree  of  freedom  only  the  temperature  or  the  TOircentration  may  be 


A 

M  °/0    O 
M'°/°  too 

Fig.  2.  —  Equilibrium  according  to  the  Phase  Rule  of  binary  alloys  whose  component  metals  are 
partially  soluble  in  each  other  in  the  solid  state. 

arbitrarily  varied.  Should  we,  for  instance,  lower  the  temperature  of  alloy  R  from 
T  to  T'  (Fig.  2)  the  composition  of  the  liquid  phase  necessarily  shifts  from  L  to  L',  and 
that  of  the  solid  phase  in  equilibrium  with  it  from  s  to  s'.  In  the  region  MSES'M'  of 
the  diagram  bounded  by  the  liquidus  and  solidus  lines,  therefore,  the  alloys  are  uni- 
variant, any  arbitrary  change  of  temperature  resulting  in  a  well-defined  change  of 
concentration  and  vice  versa.  At  E,  corresponding  to  eutectic  composition  and 
eutectic  temperature,  three  phases  are  present,  namely  two  solid  solutions  and  liquid 
alloy,  the  system  having  no  degree  of  freedom  (F  =  3  -  3  =  0).  Neither  the  tempera- 
ture nor  the  concentration  may  be  altered  without  causing  the  disappearance  of  at 
least  one  of  the  phases.  Increasing  the  temperature  must  result  in  the  disappear- 
ance of  both  solid  solutions,  the  system  becoming  bivariant,  while  lowering  it  must 
be  followed  by  the  disappearance  of  the  liquid  phase.  Again,  shifting  the  concen- 
tration to  the  left  or  right  of  E  must  yield  the  univariant  system  solid  solution  plus 
liquid.  Clearly  two  solid  phases  and  a  liquid  phase  can  only  exist  at  one  critical 
temperature  and  for  one  critical  composition  of  the  alloy;  in  the  case  of  a  eutectic 


6  LESSON    XXIV  — THE    PHASE    RULE 

alloy  these  three  phases  can  exist  only  at  its  freezing  temperature.  In  the  areas 
AM  SB  and  DM'S'C  single  homogeneous  solid  solutions  only  are  present,  that  is,  but 
one  phase  exists,  and  the  corresponding  alloys  have,  therefore,  two  degrees  of  freedom. 
Arbitrary  changes  both  of  temperature  and  composition  within  these  areas  do  not 
disturb  the  equilibrium  of  the  system.  Within  the  region  BSS'C  two  phases  occur, 
solid  solution  M  and  solid  solution  M',  the  corresponding  alloys  having,  therefore,  but 
one  degree  of  freedom.  Increasing  the  temperature  from  P  to  P',  for  instance,  must 
result  in  shifting  the  composition  of  the  solid  solutions  respectively  from  R  to  R'  and 
from  0  to  0'. 

The  Phase  Rule  Applied  to  Iron-Carbon  Alloys.  —  Since  iron-carbon  alloys  belong 
to  the  class  of  binary  alloys  the  constituents  of  which  are  partially  soluble  in  each 
other  in  the  cold,  the  application  of  the  phase  rule  to  their  equilibrium  diagram  should 
not  present  any  difficulty,  but  we  have  now  to  consider  allotropic  changes  as  well  as 
changes  of  state.  Their  possible  phases  or  metarals  are:  (1)  liquid  iron,  (2)  liquid 
solution  of  carbon  (or  Fe3C)  in  iron,  (3)  solid  solution  (austenite)  of  carbon  (or  Fe3C) 
in  gamma  iron,  (4)  solid  gamma  iron,  (5)  solid  beta  iron,  (6)  solid  alpha  iron  (ferrite), 
(7)  solid  solution  (martensite) J  of  carbon  (or  FeaC)  in  beta  iron,  (8)  solid  cementite, 
(9)  graphite,  and  possibly  others.  The  exact  nature  of  troostite  and  sorbite  being 
still  in  doubt,  they  are  not  here  classified  as  phases,  seeing  that  they  may  be,  and 
probably  are,  aggregates  of  two  or  more  phases,  unless  indeed  they  be,  according  to 
Benedicks,  emulsions  or  colloidal  solutions.  Scientists  do  not  agree,  however,  as  to 
whether  colloidal  solutions  are  or  are  not  phases,  opinions  differing  in  regard  to  their 
homogeneity.  Indeed  some  writers  like  Le  Chatelier  question  the  existence  of  colloidal 
solutions  which  they  consider  as  finely  divided  aggregates.  Pearlite  evidently  is  not 
a  phase,  but  an  aggregate  of  the  two  phases,  ferrite  and  cementite,  in  constant  pro- 
portion after  the  fashion  of  eutectic  and  eutectoid  mixtures. 

Let  us  now  apply  the  teachings  of  the  phase  rule  to  the  iron-carbon  equilibrium 
diagram  (Fig.  3).  The  number  and  kinds  of  phases  existing  at  different  temperatures,, 
and  for  different  proportions  of  the  components,  iron  and  carbon,  have  been  clearly 
indicated  and  will  be  readily  understood  in  view  of  the  foregoing  considerations. 
Above  the  liquidus  LEL'  all  alloys  are  composed  of  but  one  liquid  phase,  and  have, 
therefore,  two  degrees  of  freedom;  between  the  liquidus  and  solidus,  that  is,  in  the  region 
LSE  and  L'S'E,  two  phases  are  present,  liquid  solution  plus  solid  solution  (austenite), 
or  liquid  solution  plus  solid  Fe3C,  hence  the  corresponding  alloys  have  here  but  one 
degree  of  freedom;  alloys  of  composition  E  and  at  the  corresponding  temperature  are 
evidently  made  up  of  three  phases,  namely  liquid  solution  plus  solid  austenite  plus 
solid  cementite,  being,  therefore,  non-variant;  in  the  region  LADS  all  alloys  being  com- 
posed of  but  one  phase,  namely,  solid  austenite,  are  bivariant;  at  D  the  alloy  contains 
three  phases,  ferrite,  cementite,  and  austenite,  and  is,  therefore,  non-variant;  in  the 
area  DSS'F  two  phases  are  present,  solid  solution  (austenite)  plus  cementite,  and  the 
system  has  but  one  degree  of  freedom.  If,  as  is  often  the  case,  cementite  is  in  this 
region  decomposed  into  iron  and  graphite  the  alloys  are  for  the  time  being  non-variant, 
becoming  again  univariant  on  the  complete  disappearance  of  cementite.  In  the  region 
ABU  beta  iron  and  austenite  are  present,  the  alloys  having  in  consequence  but  one 
degree  of  freedom;  in  the  region  BCDH  alpha  ferrite  and  austenite  are  present  and 
the  alloys,  therefore,  are  univariant.  Finally  below  CDF  three  possible  cases  should 
be  considered:  (I)  the  cementite  formed  during  solidification  and  subsequent  cooling 

1  All  investigators  do  not  agree  as  to  the  homogeneity,  that  is  the  phase-like  character  of  marten- 
site,  some  still  regarding  it  as  an  aggregate. 


LESSON   XXIV  — THE    PHASE   RULE 


8  LESSON    XXIV  — THE   PHASE    RULE 

remains  unchanged,  in  which  case  the  alloy's  are  made  up  of  the  two  phases  ferrite  and 
cementite,  being,  therefore,  univariant,  their  equilibrium,  however,  as  previously  ex- 
plained, is  supposed  to  be  metastable;  (II)  the  cementite  has  been  completely  con- 
verted into  ferrite  and  graphite,  only  those  two  phases  being  present,  undoubtedly 
representing  the  stable  equilibrium  of  all  iron-carbon  alloys;  (III)  the  dissociation  of 
cementite  has  been  incomplete,  both  cementite  and  graphite  being  present,  which 
with  ferrite  give  three  phases,  the  corresponding  alloys  being  non-variant  and,  there- 
fore, their  equilibrium  unstable.  Condition  (I)  generally  prevails  in  all  grades  of  steel, 
and  is  readily  produced  in  cast  iron  by  rapid  cooling  especially  in  the  absence  of  con- 
siderable silicon,  the  resulting  alloys  being  known  as  white  cast  iron.  Condition  (II) 
never  obtains  in  steel,  but  may  be  produced  in  highly  carburized  alloys  by  very  slow 
cooling  through  and  below  solidification,  especially  in  the  presence  of  much  silicon 
and  in  the  absence  of  manganese  and  sulphur.  Condition  (III)  is  the  condition  of  the 
gray  cast  irons  of  commerce,  their  compositions  and  other  influences  prevailing  during 
their  cooling  being  such  as  to  cause  the  graphitizing  of  varying  proportions  of  cementite. 

Examination 

Describe  the  application  of  the  phase  rule  to  iron-carbon  alloys  containing  respec- 
tively 0.60,  1.25, 3.00,  and  5.00  per  cent  carbon  as  they  cool  from  the  molten  condition 
to  atmospheric  temperature. 


APPENDIX   I 

MANIPULATIONS  AND  APPARATUS 

In  the  foregoing  pages  the  author  has  described  at  length  those  apparatus  and 
manipulations  which  in  his  laboratory  he  has  found  to  yield  the  best  results.  In  the 
present  appendix  the  apparatus  and  manipulations  of  some  other  workers  are  briefly 
described. 

POLISHING  AND  POLISHING  MACHINES 

Sorby  in  his  pioneer  work  polished  his  samples  on  emery-papers  of  increasing 
fineness  followed  by  rubbing  with  tripoli,  crocus,  or  Water-of-Ayre  stone,  and  finally 
with  jeweler's  rouge.  Emery-papers  are  still  used,  but  for  quick  polishing  they 
are  often  replaced  by  emery-powders  spread  wet  on  revolving  wheels;  the  author 
has  retained  the  use  of  tripoli  powder  for  the  treatment  preceding  the  final  polish- 
ing but  others  now  prefer  specially  prepared  flour-emery  or  diamantine;  jeweler's 
rouge  is  still  widely  used  for  the  final  treatment,  although  some  prefer  specially  pre- 
pared alumina,  as  first  suggested  by  Le  Chatelier. 

In  1904  Osmond's  polishing  method  consisted  in  roughing  off  with  emery  and 
polishing  with  rouge.  Emery-papers  of  increasing  fineness  were  stretched  over  glass 
plates.  The  papers  used  were  prepared  by  mixing  with  water  levigated  "120 
minutes"  emery1  and  collecting  the  deposits  formed  at  the  end  of  increasing  periods 
of  time  in  precipitating  glasses.  The  classified  powders,  after  drying,  were  mixed 
with  a  mucilage  of  albumen  (made  up  of  72  cubic  centimeters  of  albumen  and  28 
cubic  centimeters  of  water  beaten  to  a  froth  and,  after  12  hours,  strained  through  a 
fine-meshed  sponge)  and  spread  on  paper  of  the  best  quality.  Osmond  also  pre- 
pared his  own  rouge  by  calcining  copperas  at  as  low  a  temperature  as  possible  and 
separating  the  finest  product  by  levigation.  The  rouge  was  spread  on  a  piece  of 
cloth  stretched  over  the  cast-iron  table  of  a  small  horizontal  polishing  machine  and 
used  wet. 

In  1900  Le  Chatelier's  method  of  polishing  specimens  of  iron  and  steel  previously 
rubbed  upon  emery-papers,  including  the  finest  grades,  consisted  in  rubbing  them 
successively  (1)  on  emery-paper  prepared  with  albumen,  according  to  Osmond,  with 
the  deposit  obtained  in  between  a  quarter  of  an  hour  and  one  hour  in  the  ammoniacal 
washing  of  flour-emery,  (2)  on  a  felt  disk  covered  with  some  soap  paste  prepared 
with  the  deposit  of  alumina  or  of  emery,  obtained  in  between  one  and  three  hours, 
(3)  on  a  flat  disk  made  of  wood,  metal,  or  ebonite,  covered  with  cloth,  velvet,  or 
leather  strongly  glued  upon  it;  upon  this  covering  the  soap  preparation,  obtained 
with  the  deposit  of  alumina  after  twenty-four  hours,  was  spread.  The  last  two  disks 

1  By  "120  minutes"  emery  is  meant  emery  which  has  taken  120  minutes  to  settle  in  a  vessel  of 
water  of  certain  dimensions. 

1 


APPENDIX    I  —  MANIPULATIONS    AND    APPARATUS 


were  rotated  by  some  mechanical  devices  producing  great  speed.  All  disks  must  be 
frequently  moistened  with  a  brush  or  sponge. 

According  to  Gcerens,  Le  Chatelier's  method  in  1908  consisted  in  the  use  of  (1) 
small  sheets  of  French  emery-paper,  Hubert  grades  IG  and  00  on  ground  glass  plates, 
(2)  flannel  stretched  over  glass  covered  with  "one  minute"  emery  previously  passed 
through  a  fine  sieve  (1200  meshes  per  sq.  cm.),  some  soap  solution  being  also  poured 
over  the  cloth,  (3)  a  similar  support  covered  with  "120  minutes"  emery  previously 
passed  through  a  very  fine  sieve  (2600  meshes  per  sq.  cm.)  and  washed,  and  (4)  a 
vertically  revolving  brass  disk  covered  with  flannel  and  washed  alumina.  The  fine 
alumina  mixed  with  water  and  soap  solution  may  be  sprayed  on  the  disk  by  means 
of  the  sprayer  shown  in  Figure  1. 

The  preparation  of  fine  alumina  powder  for  the  final  polishing  of  iron  and  steel 


Fig.   1.  —  Sprayer  for  emulsified   alu- 
mina.    (Gcerens.) 


Fig.  2.  —  Pipette  for  the  levigation 
of  alumina.     (Gcerens.) 


samples  was  first  described  by  Le  Chatelier  in  1900.  The  method  used  is  that  em- 
ployed by  Schloesing  for  the  analysis  of  kaolins.  The  following  description  of  Le 
Chatelier's  manipulation  is  from  Gcerens  (1908). 

The  purest  precipitated  alumina,  from  ammonia  alum,  is  passed  through  a  sieve 
of  2600  meshes  per  sq.  cm.,  and  100  grains  of  it  in  300  c.  c.  of  distilled  water  are  trit- 
urated in  a  mill  for  three  hours.  The  whole  is  then  poured  into  a  liter  flask,  well 
shaken,  and  about  200  c.  c.  pipetted  off  into  a  flask  closed  with  a  rubber  stopper. 
To  this  are  added  1800  c.  c.  of  distilled  water  and  2  c.  c.  of  concentrated  nitric  acid 
(1.4  sp.  gr.),  the  mixture  well  shaken,  and  allowed  to  settle;  the  settling  is  complete 
in  a  short  time  (about  two  hours).  The  clear  supernatant  liquid  is  siphoned  off  with 
an  S-shaped  siphon;  with  careful  manipulation  this  is  possible  to  the  extent  of  -fo 
of  the  total  amount.  The  liquid  drawn  off  is  replaced  by  distilled  water,  the  mixture 
well  shaken  several  times,  and  allowed  to  settle  again,  after  which  the  wash  water  is 


APPENDIX    I  —  MANIPULATIONS    AND    APPARATUS  3 

again  drawn  off  as  before.  This  is  repeated  three  or  four  times  more.  At  last  the 
supernatant  liquid  remains  milky  for  a  whole  day,  which  is  an  indication  of  the  per- 
fect removal  of  acid.  Finally,  distilled  water  is  added  for  the  last  time  up  to  about 
2  liters,  the  mixture  thoroughly  shaken,  and  the  alumina  separated  from  the  liquid 
in  the  apparatus  shown  in  Figure  2.  A  pipette  a  of  about  500  c.  c.  capacity  is  drawn 
out  below  to  an  opening  of  about  3  mm.  internal  diameter.  The  alumina  is  prevented 
from  clinging  by  giving  an  inclination  of  at  least  Y%  to  the  sloping  sides  of  the  tube. 
The  piece  6  is  connected  to  the  water  pump.  The  end  (of  the  pipette)  is  dipped 
into  the  vessel  containing  the  emulsified  alumina,  and  the  pipette  sucked  full,  where- 
upon the  opening  b  is  closed  with  a  screw  cap  so  far  that  one  dmpjuns  out  about  every 
fifteen  seconds.  The  material  obtained  during  the  first  quarter  of  an  hour  is  very 
heterogeneous  and  still  scratches  the  surface  of  the  section  markedly,  so  that  it  can- 
not be  used.  After  a  quarter  of  an  hour  has  expired  the  tap  is  closed  and  the  alu- 
mina allowed  to  settle  complete!}'.  After  three  hours  the  material  is  placed  in  the  flask 
A  (Fig.  1)  provided  with  a  spraying  arrangement.  Soap  solution1  is  added  and  the 
mixture  diluted  with  distilled  water.  The  material  thus  prepared  is  ready  for  use, 
and  is  suitable  for  steel  and  pig  iron.  The  residue  which  settles  in  3  to  24  hours  is 
treated  similarly,  and  serves  for  polishing  softer  materials  (iron,  copper,  etc.).  The 
portion  which  still  remains  in  suspension  after  24  hours  is  too  fine  and  is  poured  away. 

The  same  method  has  been  applied  to  commercial  flour-emery,  oxide  of  chromium, 
and  oxide  of  iron,  but  the  resulting  products  are  far  from  being  as  satisfactory  as  the 
alumina  powders. 

Revillon  has  recently  described  a  rapid  method  of  preparing  alumina  suitable  for 
polishing.  A  certain  amount  of  alumina  is  suspended  in  a  large  volume  of  water, 
well  shaken,  and  allowed  to  stand  for  five  minutes.  The  liquid  is  then  siphoned  off 
and  with  the  particles  of  alumina  still  held  in  suspension  may  be  used  for  polishing. 
To  obtain  finer  powders  15  to  20  grams  of  finely  ground  alumina  should  be  mixed 
with  one  liter  of  distilled  water,  shaken,  and  allowed  to  settle  five  minutes.  Most  of 
the  liquid  is  then  siphoned  off,  transferred  into  another  vessel,  and  allowed  to  stand 
fifteen  minutes;  the  decantation  is  repeated,  etc.,  a  clearer  liquid,  that  is  one  holding 
finer  particles  in  suspension,  being  obtained  every  fifteen  minutes.  The  final  liquid, 
from  which  no  powder  is  deposited,  may  be  used  for  the  finest  polishing,  the  inter- 
mediate products  for  rougher  work. 

Robin  has  described  the  preparation  of  alumina  powder  by  a  method  based  upon 
the  catalytic  action  of  mercury  in  causing  the  oxidation  of  pure  aluminum.  Strips 
of  aluminum  are  immersed  in  mercury  for  a  short  time  and  then  exposed  to  moist 
air  when  the  small  amount  of  mercury  they  have  absorbed  causes  the  oxidation  of 
the  metal,  growth  of  A1203  taking  place,  the  increase  of  which  is  visible  with  the  naked 
eye.  This  alumina  can  be  readily  detached  and  as  a  fine  powder  may  be  used  for 
polishing.  Robin  claimed  for  his  method  the  advantages  of  greater  simplicity  and 
lower  cost. 

In  1900  Stead  recommended  for  polishing  iron  and  steel  samples  the  use  of  emery- 
papers,  Hubert  grades  Nos.  0,  00,  and  000,  followed  by  rubbing  with  one  grain  of 
diamantine  powder2  spread  wet  over  a  smooth  black  cloth  and,  for  final  treatment, 
gold  rouge  used  dry  on  chamois  leather  or,  for  finer  structures,  wet  on  parchment  or 

1  The  .soap  solution  is  prepared  by  dissolving  pure  (Venetian)  soap  in  hot  water  and  filtering 
through  a  filter  paper  into  a  flask.    After  cooling  the  solution  should  he  sirupy. 

2  Diamantine  powder  consists  of  pure  alumina  and  is  used  by  jewelers  for  polishing  steel. 


4  APPENDIX    I  —  MANIPULATIONS    AND    APPARATUS 

kid  leather.    He  used  a  simple,  hand  polishing  machine  in  which  one  block  at  a  time 
was  made  to  rotate  horizontally  (Fig.  3). 

A  foot  polishing  machine  also  designed  by  Stead  is  shown  in  Figure  4  and  a 


•3 

- 

CO 


"o 


CO 

cab 


larger  one  to  be  run  by  power  in  Figure  5.  In  these  machines  brass  disks  carry- 
ing conical  wooden  blocks  are  attached  to  vertical  spindles  and  driven  from  below. 
Emery-papers  are  fastened  to  some  of  these  blocks  by  means  of  brass  rings  slipping 


APPENDIX  I  — MANIPULATIONS  AND  APPARATUS  5 

over  them  while  others  are  covered  with  cloth  in  a  similar  way.  Clamps  are  provided 
for  holding  the  samples  against  the  revolving  disks.  The  central  vessel  contains  the 
water  needed  for  wet  polishing,  a  small  tap  projecting  over  each  disk.  The  excess 
water  is  caught  by  brass  water  guards  and  discharged  into  a  trough  below  the  level 
of  the  disks.  These  machines  are  made  by  Carling  and  Sons  of  Middlesbrough,  Eng- 
land. 

Martens,  according  to  Gcerens  (1908),  uses  vertically  rotating  disks  upon  which 


Fig.  4.  —  Foot  power  polishing  machine. 
(Stead.) 


Fig.  5.  —  Multiple  polishing  machine.     (Stead.) 


are  pasted  emery-papers,  Hubert  brand,  grades  3,  2,  1G,  1M,  IF,  0,  and  00  and,  for 
final  treatment,  levigated  jeweler's  rouge  on  cloth.  The  disks  make  400  revolutions 
per  minute.  The  average  time  needed  to  polish  a  specimen  varies  between  \Yi  and 
2  hours. 

Gulliver  (1908)  recommends  for  polishing  the  use  of  emery-papers  grades  No.  1, 
0,  and  00  on  hard  wood  or  plate  glass  and  for  final  treatment  the  finest  rouge  or  dia- 
mantme  powder  on  cloth  stretched  over  hard  wood. 

The  polishing  machines  shown  in  Figures  6  and  7  are  made  by  P.  F.  Dujardin  of 
Dusseldorf.  It  will  be  noted  that  one  side  only  of  the  disks  is  utilized.  A  machine 
like  the  one  of  Figure  7  is  also  made  for  belt  driving. 


6 


APPENDIX    I  —  MANIPULATIONS    AND    APPARATUS 


Sexton  describes  the  polishing  machine  Figure  8,  made  by  Baird  and  Tatlock. 
Its  construction  is  obvious. 

A  simple  polishing  machine  consisting  of  an  horizontally  revolving  disk  (Fig.  9) 
was  described  in  1899  by  Ewing  and  Rosenhain.  A  is  the  spindle  of  an  electric  motor 
carrying  a  small  driving  disk  B,  fitted  with  a  rubber  ring  to  increase  the  driving  fric- 


Fig.  G.  —  Foot  power  polishing  machine.     (P.  F.  Dujardin  and  Co.) 

tion.  The  polishing  disk  C  has  a  vertical  axis  running  in  a  bearing  on  the  casting  D. 
The  under  side  of  the  polishing  disk  bears  upon  the  driving  wheel  B  and  takes  motion 
from  it. 

A.  Kingsbury  in  1910  described  his  polishing  method.  He  prepares  his  support- 
ing blocks  by  pouring  paraffin  on  brass  disks.  After  solidifying  these  paraffin 
blocks  which  are  about  J/£  inch  thick  and  8  inches  in  diameter  have  their  upper  face 
dressed  flat.  They  are  made  to  rotate  horizontally  in  a  suitable  machine  and  upon 
them  emery  of  increasing  fineness  and  finally  rouge  are  used  in  succession.  The 


APPENDIX    I  —  MANIPULATIONS    AND    APPARATUS  7 

speed  of  the  polishing  machine  is  200  revolutions  per  minute.     The  time  needed  to 
polish  a  sample  of  ordinary  steel  is  given  as  fifteen  minutes. 

C.  Campbell  in  1902  described  the  polishing  operation  as  consisting  in  rubbing  the 
sample,  previously  filed  smooth,  successively  on  emery-cloth,  grades  0  and  00,  and  on 
French  emery-papers,  grades  0,  00,  000,  and  0000.  The  specimen  is  then  polished  on 


Fig.  7.  —  Polishing  motor.     (P.  F.  Dujardin  and  Co.) 

broadcloth  or  chamois  leather  with  well  washed  rouge  and  water.     Some  workers, 
the  writer  says,  use  an  intermediate  stage  with  diamantine  powder. 

C.  H.  Risdale  in  1899  described  his  polishing  operation  as  consisting  in  (1)  rough 
filing,  (2)  fine  filing,  (3)  rubbing  with  rough  commercial  emery-cloth  stretched  on  a 
board,  (4)  rubbing  with  fine  emery-cloth  stretched  on  a  board,  (5)  rubbing  on  fine 
specially  prepared  paper  on  disks  of  Stead's  polishing  machine,  (6)  rubbing  on  dia- 
mantine on  cloth  stretched  on  disks  of  Stead's  machine,  (7)  rubbing  on  rouge  on 


8 


APPENDIX    I  —  MANIPULATIONS    AND    APPARATUS 


washed  leather  similarly  mounted  or,  for  very  fine  work,  on  rouge  on  wetted  parch- 
ment. 

Guillet  in  1907  recommended  for  polishing  two  carborundum  wheels  and  two 
suitably  selected  emery-papers  and,  for  final  treatment,  alumina  on  cloth  stretched 
over  a  revolving  disk.  He  places  smooth  sheets  of  zinc  between  the  wooden  disks  of 
his  polishing  machines  and  the  polishing  cloths. 

In  1901  Arnold  described  as  follows  a  quick  polishing  and  etching  method:  "Take 


Fig.  8.  —  Polishing  machine.     (Baird  and  Tatlock.) 


Fig.  9.  —  Polishing  machine.     (Ewing  and  Rosenhain.) 


two  pieces  of  hard  wood,  12"  x  9"  x  1",  planed  dead  smooth  on  one  side;  then  by 
means  of  liquid  glue  evenly  attach  to  the  smooth  faces  two  sheets  of  the  London 
Emery  Works  Go's  atlas  cloth  No.  0.  Allow  the  glue  to  set  under  strong  pressure. 
Next,  by  means  of  a  smooth  piece  of  steel,  rub  off  from  one  of  the  blocks  as  much  as 
possible  of  the  detachable  emery.  This  is  No.  2  block,  the  other,  necessarily,  No.  1 
block. 

"The  steel  section,  say  J^  inch  thick  and  3/2  inch  diameter,  is  rubbed  for  one  minute 


APPENDIX    I  —  MANIPULATIONS   AND   APPARATUS  9 

on  No.  1  block,  the  motion  being  straight  and  not  circular;  then,  for  the  same  time 
and  in  the  same  manner  rub  on  No.  2  block.  Next  place  the  bright  but  visibly 
scratched  sections  in  a  glass  etching  dish  3"  X  1"  X  H">  and  cover  the  steel  with  nitric 
acid  sp.  gr.  1.20. 

"Watch  closely  until  in  a  few  seconds  the  evolved  gases  adhering  to  the  section 
change  from  pale  to  deep  brown  and  effervescence  ensues.  Then,  under  the  tap, 
quickly  wash  away  the  acid  and  for  a  minute  immerse  the  piece  in  a  second  dish  con- 
taining rectified  methylated  spirits.  Dry  the  section  by  pressing  it  several  times  on 
a  soft  folded  linen  handkerchief,  when  it  will  be  ready  for  examination.  The  struc- 
ture will  be  clearly  exhibited,  the  innumerable  fine  scratches^  visible  before  etching 
having  virtually  vanished." 

DEVELOPMENT  OF  THE  STRUCTURES 

The  methods  which,  in  common  with  many  workers,  the  author  has  found  most 
satisfactory  for  revealing  the  structure  of  polished  iron  and  steel  specimens  have  been 
described  in  these  pages.  They  include  etching  with  concentrated  nitric  acid,  with 
very  dilute  alcoholic  solutions  of  nitric  acid  aqd  of  picric  acid  (Lesson  III),  with 
sodium  picrate  and  ammonium  oxalate  (Lesson  V),  and  with  the  Kourbatoff  reagent 
(Lesson  XIII).  Other  methods  have  been  used  that  should  be  mentioned. 

Polishing  in  Relief.  —  So-called  relief  polishing  has  been  used  successfully  by 
Sorby,  Martens,  Behrens,  and  especially  by  Osmond.  It  consists  in  rubbing  the 
specimen  on  a  soft,  yielding  support  with  some  suitable  polishing  powder,  the  softest 
constituents  being,  so  to  speak,  dug  out,  leaving  the  harder  ones  standing  in  relief. 
These  differences  of  level  make  it  possible  to  distinguish  the  constituents  under  the 
microscope  without  further  treatment.  It  is  evident  that  only  those  samples  which 
are  made  up  of  constituents  differing  much  in  hardness  can  be  so  treated.  The  free 
cementite  of  hyper-eutectoid  steel  or  of  white  cast  iron,  for  instance,  can  be  made  to 
stand  strongly  in  relief  because  it  is  so  much  harder  than  the  accompanying  pearlite 
or  other  constituents. 

Osmond  polishes  his  samples  on  a  damp  piece  of  parchment  stretched  over  a  piece 
of  well-planed  wood.  It  is  sprinkled  with  rouge  which  is  rubbed  strongly  on  the 
parchment.  The  block  is  then  put  under  the  tap  and  washed  so  that  only  those  part- 
icles of  rouge  that  have  found  their  way  into  the  pores  of  the  parchment  are  retained. 
To  distinguish  between  raised  portions  and  cavities  the  luminous  rays  are  strongly 
diaphragmed  and  the  objective  placed  a  little  below  the  focusing  point,  is  slowly 
raised.  The  reliefs,  which  at  first  appear  brilliant  and  yellowish  on  a  relatively 
darker  ground,  gradually  become  dark  on  a  bright  ground;  the  cavities  present  in- 
verse appearances  so  perfectly  that  two  photographs  of  the  same  preparation,  taken 
one  a  little  below  and  the  other  a  little  above  the  mean  focusing  point,  are  almost 
positive  and  negative  to  one  another. 

Polish-Attack.  —  For  many  years  Osmond  obtained  his  best  preparations  by  a 
combined  polishing  and  etching  method  (polissage-attague)  consisting  in  rubbing  the 
polished  sample  upon  a  piece  of  parchment  covered  with  some  aqueous  extract  of 
liquorice  root,  with  the  addition  of  precipitated  calcium  sulphate.  In  1899  Osmond 
and  Cartaud  recommended  replacing  the  extract  of  liquorice  by  a  diluted  solution  of 
nitrate  of  ammonium  (2  parts  by  weight  of  the  crystallized  salt  to  100  parts  of 
water).  A  piece  of  parchment  spread  tightly  over  a  smooth  board  is  soaked  with  the 


10  APPENDIX    I  —  MANIPULATIONS  AND  APPARATUS 

solution  and  the  specimen  rubbed  upon  it  until  sufficiently  etched.  It  is  not  necessary 
to  add  any  sulphate  of  calcium. 

Etching.  —  Sorby  etched  his  specimens  with  very  dilute  solutions  of  nitric  acid  in 
water  and  this  reagent  was  widely  used  for  many  years  by  other  metallographists. 
The  water  has  now  been  replaced  by  absolute  alcohol  (Lesson  III,  page  7).  Le  Chate- 
lier  has  mentioned  the  use  of  glycerine  as  a  satisfactory  non-oxidizing  vehicle  for 
nitric  as  well  as  for  picric  and  hydrochloric  acid. 

Osmond,  the  author  believes,  was  the  first  to  use  tincture  of  iodine.  This  tinc- 
ture is  applied  in  the  proportion  of  one  drop  per  square  centimeter  of  surface  and 
allowed  to  act  until  it  is  decolorized,  the  treatment  being  repeated  after  examina- 
tion if  needed.  Le  Chatelier  recommends  applying  the  tincture  with  the  tip  of  the 
finger  and  gently  rubbing  the  specimen. 

Stead  uses  a  solution  made  up  of  1.25  grains  of  iodine,  1.25  grains  of  iodide  of 
potassium,  1.25  grains  of  water  and  alcohol  to  make  up  100  c.  c.  After  the  iodine  has 
lost  its  color  the  sample  should  be  washed  in  water,  then  in  alcohol,  and  finally  dried 
in  a  blast  of  hot  air. 

Martens  and  Heyn  in  1904  recommended  the  use  (1)  of  an  etching  solution  con- 
taining one  part  of  hv-Jrcchloric  acid  (1.19  sp.  gr.)  and  100  parts  of  absolute  alcohol, 


Fig.  10.  —  Arrangement  for  electrolytic  etching. 

and  (2)  of  one  part  of  hydrochloric  acid  in  500  parts  of  water  with  the  assistance  of 
the  electric  current. 

Heyn  used  a  solution  of  double  chloride  of  copper  and  ammonium  containing  12 
grains  of  the  salt  and  100  grains  of  distilled  water. 

To  distinguish  with  certainty  between  iron  phosphide  and  cementite,  Matweieff 
recommends  neutral  sodium  picrate  washed  several  times  with  distilled  water  to 
eliminate  the  excess  of  picric  acid  or  of  sodium  that  might  be  present.  The  sample 
is  immersed  in  the  boiling  solution  for  20  minutes,  a  treatment  by  which  the  iron 
phosphide  is  strongly  attacked  while  the  cementite  and  pearlite  remain  unaffected. 

For  etching  austenite  and  martensite  Robin  recommends  the  use  of  a  saturated 
solution  of  picric  acid  in  alcohol,  an  immersion  of  thirty  seconds  to  one  minute,  wash- 
ing with  water  without  touching  the  specimen  and  drying  by  air  blast  or  simply  in 
air.  Films  of  various  tints  are  formed,  ferrite  remaining  uncolored. 

Le  Chatelier  has  used  bitartrate  of  potassium  as  an  etching  reagent.  It  leaves 
cementite  and  pearlite  uncolored,  while  it  imparts  a  dirty  coloration  to  ferrite. 

The  same  author  has  described  the  use  of  a  freshly  prepared  reagent  made  up  of 
equal  parts  of  a  solution  containing  50  per  cent  of  soda  and  of  a  solution  containing 
10  per  cent  of  lead  nitrate.  Cementite  is  quickly  colored  by  it  while  the  phosphides 
and  especially  the  silicides  are  also  attacked.  The  reagent  is  recommended  for  highly 
carburized  metals.  Medium  high  carbon  steels  of  great  purity  are  not  affected  by 
this  solution,  but  when  impure,  the  pearlite  is  energetically  acted  upon,  probably 
because  of  the  presence  of  impurity  in  that  constituent. 


APPENDIX   I  —  MANIPULATIONS  AND  APPARATUS  11 

Le  Chatelier  has  also  mentioned  the  use  of  a  solution  of  10  per  cent  gaseous  hy- 
drochloric acid  in  absolute  alcohol  to  which  is  added  5  per  cent  of  cupric  chloride  for 
annealed  steels  and  one  per  cent  of  the  same  salt  for  hardened  steels.  Ferrite  and 
cementite  are  not  colored,  martensite  very  little,  austenite  a  little  more,  troostite 
and  sorbite  decidedly. 

Hilpert  and  Colver-Glauert  have  described  the  use  of  sulphurous  acid  for  non- 
pearlitic  steels  and  for  pig  iron.  A  saturated  solution  of  sulphur  dioxide  in  water  is 
prepared  and  3  or  4  per  cent  of  tliat  solution  in  water  used.  The  time  of  etching 
varies  between  seven  seconds  and  one  minute.  Alcohol  may  be  substituted  for  water 
in  which  case  the  etching  lasts  several  minutes.  The  treatment  causes  the  deposi- 
tion of  layers  of  iron  sulphide  of  different  thickness  and,  therefore,  of  different  colors, 
on  the  various  constituents. 

Electrolytic  Etching.  —  Lc  Chatelier  was  one  of  the  first  to  advocate  the  use  of 
the  electric  current  in  order  to  obtain  a  more  uniform  action  in  etching  iron  and  steel 
samples.  Sheet  lead  may  be  used  for  the  positive  electrode,  and,  as  electrolyte,  a  10 
per  cent  solution  of  chloride  or  sulphate  of  ammonium  gives  good  results.  The  cur- 
rent needed  varies  between  0.001  and  0.01  amperes  per  square  centimeter. 

Electrolytic  etching  has  been  described  by  Cavalier  (1909).  A  few  cubic  centi- 
meters of  the  electrolyte  are  placed  in  a  platinum  dish  C  (Fig.  10)  connected  with 
one  pole  of  the  battery  P;  the  specimen  E  connected  with  the  other  pole  is  placed 
in  the  solution,  a  piece  of  filter  paper  A  being  inserted  between  the  dish  and  the 
polished  surface  of  the  specimen.  The  current  is  regulated  through  the  rheostat  R. 
Four  or  five  volts  are  required  with  an  intensity  of  0.001  to  0.01  amperes  per  square 
centimeter.  The  attack  lasts  from  a  few  seconds  to  a  few  minutes. 

Heat  Tinting.  —  Heat  tinting  as  a  means  of  imparting  different  appearances  to 
the  various  constituents  of  iron  and  steel  was  first  used  by  Behrens  and  Martens  and 
later,  with  much  success,  by  Stead.  When  a  polished  piece  of  iron  or  steel  is  heated 
in  an  oxidizing  atmosphere  oxidized  films  are  formed,  the  color  of  which  varies  with 
the  thickness,  that  is,  with  the  temperature  and  duration  of  treatment.  It  is  also 
found  that  the  various  constituents  are  differently  colored  because  oxidized  at  dif- 
ferent speeds.  According  to  Stead  the  metal  should  be  first  well  rubbed  with  a  piece 
of  linen  or  chamois  leather  and  placed  on  an  iron  plate  heated  by  a  Bunsen  burner. 
It  is  best  to  heat  gradually  and  examine  periodically  under  the  microscope  and  stop 
when  the  structure  appears  to  be  most  perfectly  colored.  After  each  heating  the 
section  may  be  placed  in  a  dish  of  mercury  so  as  to  cool  it  rapidly  and  check  further 
oxidation.  The  oxidized  films  assume  in  succession  the  following  tints  as  they  in- 
crease in  thickness:  pale  yellow,  yellow,  brown,  purple,  blue,  and  steel  gray.  The 
method  is  especially  useful  for  identifying  phosphides,  sulphides,  and  carbides  in  cast 
iron  and  for  detecting  the  more  highly  phosphorized  portions  of  iron  and  steel.  Free 
cementite  colors  less  readily  than  iron  but  more  rapidly  than  phosphide  of  iron.  Iron 
containing  phosphorous  in  solid  solution  colors  more  rapidly  than  pure  iron  or  than 
iron  containing  less  phosphorus. 

Hot  Etching.  —  Steel  while  at  a  high  temperature  (red  heat)  has  been  etched 

(1)  by  Saniter  in  molten  calcium  chloride  heated  to  the  desired  temperature,  and 

(2)  by  Baykoff  in  a  current  of  gaseous  hydrochloric  acid. 

Washing  and  Drying.  —  After  removing  the  specimens  from  the  etching  bath,  the 
author  washes  them  in  alcohol  and  dries  them  in  an  air  blast.  They  are  then  rubbed 
once  or  twice  very  gently  on  a  block  covered  with  a  fine  piece  of  chamois  skin  and 


12 


APPENDIX    I  —  MANIPULATIONS   AND   APPARATUS 


carefully  kept  free  from  dust.  Washing  in  water,  in  caustic  potash,  in  lime  water, 
and  in  ether  has  also  been  recommended  as  well  as  the  use  of  fine  linen  cloth  and  of 
a  hot  blast  for  drying. 

Preserving.  —  The  author  preserves  his  etched  specimens  in  dessicators  and  in 
air-tight  cabinets. 

Several  protective  coatings  have  been  described.  Stead  covers  them  with  paraf- 
fin wax  dissolved  in  benzole,  which  is  removed  by  wiping  with  a  clean  linen  rag  mois- 
tened with  benzole,  when  it  is  desired  to  examine  the  specimens.  Le  Chatelier  applies 
a  coating  of  "zapon,"  a  solution  of  gun  cotton  in  amyl  acetate  sufficiently  transparent 
to  allow  examination  with  the  highest  powers. 

By  keeping  the  specimens  in  mercury  their  tarnishing  should  be  effectively  pre- 
vented while  they  would  be  at  all  times  accessible  for  immediate  examination.  Nor 
should  this  scheme  call  for  the  use  of  a  large  amount  of  mercury  nor  for  much  space; 
flat  glass  trays  might  be  used  containing  just  enough  mercury  to  cover  their  smooth 
bottom  and  the  specimens  placed  in  them  polished  face  down.  In  this  way  a  large 


.Glass  slide 


Brass  ri 


Class 


Fig.  11.  —  Stead's  mounting  device. 
(C.  H.  Desch's  Metallography.) 


Fig.  12.  —  Gulliver's  mounting  device. 


number  of  samples  could  be  preserved  in  a  small  place  and  in  a  small  quantity  of 
mercury.  In  a  tray  measuring  12  by  12  inches,  for  instance,  nearly  200  samples  of 
ordinary  size  (^  to  %  inch  in  diameter)  could  be  kept. 


MOUNTING  AND  MOUNTING   DEVICES 

The  author's  special  holders  for  placing  the  prepared  samples  on  the  stage  of  the 
microscope  have  been  described  (see  Apparatus  for  the  Metallographic  Laboratory, 
page  7).  Other  methods  have  been  used  and  are  still  employed  by  some  workers, 
namely  (1)  mountir. "  in  some  plastic  material,  and  (2)  the  use  of  leveling  stages. 

Plastic  Mounting.  —  Osmond  mounts  his  specimens  by  embedding  them  in  a 
little  soft  wax  placed  upon  a  glass  plate.  The  leveling  is  managed  by  means  of  two 
pieces  of  glass  tube  of  equal  height,  one  on  each  side  of  the  sample. 

Stead  places  the  specimens  polished  face  down  on  a  piece  of  plate  glass  (Fig.  11) 
and  surrounds  them  with  brass  cylinders  accurately  turned.  A  piece  of  plastic  wax 
is  stuck  upon  the  center  of  a  glass  microscope  slide  and  is  then  pressed  upon  the  sec- 
tion till  the  glass  slide  comes  in  contact  with  the  brass  ring.  The  specimen  adheres 
to  the  wax  and  the  mounting  is  complete. 

Gulliver  (1908)  describes  the  device  (Fig.  12)  for  mounting  specimens.    It  consists 


APPENDIX    I  —  MANIPULATIONS   AND   APPARATUS 


13 


of  a  circular  ring  faced  on  its  upper  surface  A,  and  screwed  internally  at  B  to  fit  the 
foot,  of  which  the  upper  end  C  is  also  faced.  The  distance  between  the  parallel  faces 
A  and  C  can  thus  be  adjusted.  The  specimen  is  placed  at  D  and  a  glass  slide  E  with 
some  soft  modeling  clay  or  wax  is  pressed  upon  it  until  the  glass  touches  the  ring 
at  AA. 

Mechanical  mounting  devices  working  on  the  principle  of  the  microtome  have 
also  been  used.  They  have  been  described  by  M.  A.  Richards:  "Projecting  from  a 
cylindrical  metal  base  three  inches  in  diameter,  is  a  threaded  upright  three  and  one- 


Fig.  13.  —  Watson  and  Sons'  mounting 
device. 


Fig.  14.  —  Watson  and  Sons' 
leveling  stage. 


Fig.  15.  —  Huntington's  leveling  stage. 

half  inches  in  diameter.  A  cylindrical  nut  or  collar  three  inches  high  and  two  and 
one-half  inches  outside  diameter  screws  on  the  threaded  uprigL  A  small  circle  of 
chamois  skin  is  placed  on  the  top  of  the  thread  upright  to  protect- the  etched  face  of 
the  micro-section.  To  mount  a  section,  place  it  face  down  on  the  chamois  skin,  press 
upon  the  upper  projecting  portion  a  lump  of  beeswax,  and  upon  this  place  the  ground 
glass  (ground  surface  down).  A  few  revolutions  of  the  collar  will  cause  the  glass  to 
rest  upon  the  upper  edge  of  the  collar,  and  the  adhesion  of  the  glass  and  beeswax  to 
the  specimen  may  be  made  complete  by  slowly  turning  the  collar  down  with  one  hand 
while  keeping  the  glass  base  in  close  contact  with  the  collar-top  with  the  other  hand. 
In  this  manner,  no  matter  how  irregular  the  section,  the  parallelism  of  the  etched 
surface  and  the  glass  base  may  be  very  quickly  and  accurately  obtained." 


14 


APPENDIX    I  —  MANIPULATIONS   AND   APPARATUS 


The  mounting  device  (Fig.  13)  is  constructed  by  Watson  and  Sons.  It  consists  of 
two  horizontal  plates,  the  upper  one  being  capable  of  vertical  movement  but  always 
remaining  parallel  to  the  lower  one.  The  specimen  is  placed  with  its  polished  surface 
on  the  lower  plate,  and  the  upper  plate  carrying  a  glass  slip  to  which  some  suitable 
clay  or  wax  is  attached  is  lowered  into  contact. 

Leveling  Stages.  —  The  leveling  stage  (Fig.  14)  is  constructed  by  Watson  and 


Fig.  1G.  —  Le  Chatelier's  inverted  metallurgical  microscope. 
Early  form. 


r        1°    0  Ol 

V 

f                    l                1 

G 


1  —  »                      .  — 

K 

\ 

F                               I  1 

I 

"~1  i 



_r 

Fig.  17.  —  Le  Chatelier's  inverted  metallurgical  microscope. 

Sons,  London.  The  specimen  is  held  by  two  rotating  jaws  and  can  bo  leveled  by 
means  of  the  screws  A  and  B  BI. 

Professor  A.  K.  Huntington  devised  the  leveling  stage  shown  in  Figure  15.  It  is 
provided  with  a  ball  and  socket  joint  for  leveling,  permitting  the  placing  of  the  sam- 
ple in  any  position. 

Other  forms  of  leveling  stages  are  shown  in  some  of  the  illustrations  in  the  follow- 
ing pages  as  part  of  some  metallurgical  microscopes. 


APPENDIX    I  —  MANIPULATIONS   AND   APPARATUS 


15 


16 


APPENDIX    I  —  MANIPULATIONS   AND   APPARATUS 


METALLURGICAL  MICROSCOPES 

The  microscopes  and  accessories  used  by  the  author  have  been  fully  described. 
In  the  following  pages  instruments  used  by  some  other  workers  or  described  by  them, 
as  well  as  those  manufactured  by  well-known  makers,  are  mentioned. 

Le  Chatelier.  —  In  1897  Le  Chatelier  devised  an  inverted  microscope  which  later 


Fig.  19.  —  Le  Chatelier's  inverted  metallurgical  microscope. 


Fig.  20.  —  Device  for  placing 
specimens  on  the  stage  of 
the  Le  Chatelier  micro- 
scope in  a  fixed  position. 
(Le  Grix.) 

he  greatly  improved  and  which  is  now  constructed  with  unimportant  modifications 
by  several  microscope  makers.  An  early  form  of  Le  Chatelier's  instrument  is  shown 
in  Figure  16  and  its  more  recent  construction  in  Figures  17  and  18.  The  objective 
B  (Fig.  17)  is  directed  upwards  while  the  eye-piece  0,  placed  horizontally,  receives 
the  image  by  the  reflection  of  a  totally  reflecting  prism  F  placed  below  the  objective. 
The  prism  F  may  be  rotated  by  means  of  the  milled  head  P  and  the  light  reflected  by 
the  objective  turned  at  will  into  the  tube  G  and  the  eye-piece  O  for  visual  examination 


APPENDIX    I  —  MANIPULATIONS   AND   APPARATUS 


17 


or  into  another  tube  connected  with  a  camera  for  photographing  (Fig.  18).  The 
light  is  condensed  by  the  lens  A  and,  being  deflected  at  right  angles  by  the  prism  /, 
passes  through  the  objective  B  and  reaches  the  object  M  placed  on  the  stage  E.  In 
case  the  light  is  placed  at  a  higher  level  than  the  condensing  lens  A,  it  must  be  re- 
ceived by  a  totally  reflecting  prism  H  which  directs  it  into  the  condenser  A.  D  is  a 
diaphragm  placed  at  the  principal  focus  of  the  complex  optical  system  composed  of 
the  objective  R,  the  illuminating  prism  J,  and  the  lens  A,  The  opening  as  well  as  the 
position  of  the  diaphragm  may  be  altered.  Another  diaphragm  placed  at  7  affords 
a  means  of  stopping  the  light  which  would  fall  upon  parts  of  the  preparation  outside 
of  the  portion  examined  and  which  would  increase  the  blur  resulting  from  the  reflec- 
tion of  useless  rays  by  the  back  lenses  of  the  objective.  In  the  early  construction  of 
this  instrument  when  the  object  was  to  be  photographed  the  prism  F  was  withdrawn 
from  the  path  of  light  and  the  image  allowed  to  form  on  a  photographic  plate  placed 
below  (Fig.  16). 

A  slightly  different  construction  is  shown  in  Figure  19.     For  photographic  pur- 
poses the  image  forms  on  a  plate  placed  in  a  holder  rigidly  connected  with  the  instru- 


Fig.  21.  —  Inverted  metallurgical  microscope  constructed  by  E.  Leitz. 

ment,  no  eye-piece  being  used.  As  the  distance  between  the  photographic  plate  and 
the  objective  is  short,  very  small  photomicrographs  are  obtained,  which  must  gen- 
erally be  subsequently  enlarged.  Z  is  a  plate  carrying  an  eye-piece  for  use  with  the 
long  bellows  camera  (Fig.  18).  The  Le  Chatelier  microscopes  are  constructed  by 
Ph.  Pellin  of  Paris. 

In  order  to  be  able  to  examine  identical  portions  of  the  same  specimen  at  different 
times  with  the  Le  Chatelier  microscope,  Le  Grix  (1907)  suggested  the  arrangement 
shown  in  Figure  20.  A  circular  metallic  disk  with  rectangular  opening  RR'  and  carry- 
ing two  pointed  stops  A  and  B  is  fitted  to  the  stage.  A  file  mark  E  is  made  in 
the  specimen  M,  which  is  then  placed  on  the  stage  so  that  the  stop  A  enters  the 
groove  E  while  the  specimen  presses  against  the  other  stop  B,  in  this  way  securing 
a  constant  position  for  the  object. 

Ernst  Leitz.— A  slightly  modified  form  (Fig.  21)  of  the  Le  Chatelier  inverted 
microscope  is  made  by  Ernst  Leitz  of  Wetzlar,  Germany.  The  modifications  were 
suggested  by  Guertler.  The  stage  and  illuminating  appliances  are  shown  on  a  larger 
scale  in  Figure  22. 

The  same  maker  also  manufactures  the  microscope  shown  in  Figures  23  and  24 


18 


APPENDIX    I  —  MANIPULATIONS   AND   APPARATUS 


designed  by  W.  Campbell.    The  stage  can  be  removed  and  the  upper  part  of  the  in- 
strument attached  to  the  base  for  the  examination  of  large  surfaces. 

P.  F.  Dujardin.  —  P.  F.  Dujarclin  and  Co.  of  Diisseldorf  construct  a  Le  Chatelier 


Fig.  22.  —  Inverted  metallurgical  microscope  constructed  by  E.  Leitz. 


Fig.    23.  —  Metallurgical    microscope    con- 
structed by  E.  Leitz. 


Fig.  24.  —  Metallurgical  microscope 
constructed  by  E.  Leitz. 


inverted  microscope  as  shown  in  Figure  25.    They  also  make  the  microscope  (Fig.  26) 
in  which  the  vertical  illuminator  carries  its  own  source  of  light  and  condenser. 

C.  Reichert.  —  The  metallurgical  microscope  (Fig.  27)  designed  by  Professor 
Rejto  is  made  by  C.  Reichert  of  Vienna.  The  position  of  the  vertical  illuminator 
immediately  below  the  eye-piece  should  be  noted.  The  stage  is  provided  with  a  level- 


APPENDIX    I  —  MANIPULATIONS   AND   APPARATUS 


19 


ing  mechanism.  The  same  maker  manufactures  an  inverted  Le  Chatelier  microscope 
as  shown  in  Figure  28.  According  to  Desch,  in  this  microscope,  two  right-angled 
prisms  are  cemented  together  to  form  the  cube  P  (Fig.  29).  The  upper  prism  is  sil- 


vered over  an  elliptical  area,  as  shown  by  the  central  dark  line.  A  portion  of  the 
light  proceeding  from  the  mirror  M  passes  through  the  clear  portion  of  the  glass  cube 
P  and  falls  upon  the  object  S.  The  light  reflected  back  by  the  object  upon  striking 


20 


APPENDIX   I  —  MANIPULATIONS  AND  APPARATUS 


the  silvered  portion  of  the  prism  is  deflected  at  right  angles  into  the  tube  0  which 
conducts  it  to  the  eye  or  to  a  photographic  plate. 

R .  Fuess.  —  A  metallurgical  microscope  practically  identical  in  construction  to 
the  Le  Chatelier  inverted  instrument  is  made  by  R.  Fuess  of  Steglitz,  near  Berlin. 

Robin.  —  The  microscope  and  photographic  attachment  shown  in  Figure  30  was 


Fig.  26.  —  Metallurgical  microscope  constructed  by  P.  F.  Dujardin 

and  Co. 


ERT,WIEN. 


Fig.  27.  —  Metallurgical  microscope  con 
structed  by  C.  Reichert. 


designed  by  Robin.  Visual  examination  is  possible  only  on  the  screen  of  the  camera. 
The  stage  consisting  of  a  smooth  disk  is  tilting  and  the  specimen  is  fastened  upon  it 
with  wax.  To  secure  an  accurately  horizontal  position  of  the  polished  surface,  a 
plug  with  a  perfectly  flat  surface  is  screwed  into  the  microscope  in  place  of  the  ob- 
jective and  the  stage  raised  until  the  specimen  coming  in  contact  with  the  plug,  the 
latter  through  gentle  pressure  causes  the  polished  surface  to  assume  a  horizontal 
position.  The  plug  is  then  removed  and  the  objective  inserted. 


APPENDIX    I  —  MANIPULATIONS   AND   APPARATUS 


21 


Martens.  — The  Martens  metallurgical  microscope  (1899)  made  by  Zeiss  of  Jena  is 
shown  in  Figure  31.    It  can  be  used  horizontally  only,  the  tube  is  very  wide  and 


Fig.  28.  —  Inverted  metallurgical  microscope  constructed  by  C.  Reichert. 


Fig.   29.  —  Illuminating    prisms     of 
Reichert's  inverted  microscope. 


Fig.  30.  —  Metallurgical  microscope  designed  by  Robin. 

the  vertical,  mechanical  stage  is  provided  with  both  coarse  and  fine  adjustments  Y 
and  Z  and  with  leveling  screws  aa.    The  flexible  connection  /  permits  the  focusing 


22 


APPENDIX    I  —  MANIPULATIONS   AND   APPARATUS 


of  the  object  from  the  camera  screen.  The  instrument  is  designed  especially  for 
photography. 

A  complete  Zeiss  equipment  including  a  large'  electric  arc  lamp  is  shown  in 
Figure  32.  It  will  be  noted  that  the  mounting  of  the  camera  is  entirely  separate 
from  that  of  the  other  parts. 

Martens  also  designed  the  ball-jointed  microscope  (Fig.  33)  which  he  used  prin- 
cipally for  observing  the  progress  of  etching. 

Rosenhain.  —  The  microscope  shown  in  Figure  34  was  constructed  by  R.  and  J. 
Beck  for  Rosenhain.  The  stage  is  mechanical  and  provided  with  coarse  and  fine 


Fig.  31.  —  Martens  metallurgical  microscope. 

adjustments,  and  all  controlling  heads  are  placed  beneath.  Appliances  are  provided 
for  various  kinds  of  illumination.  The  necessary  alteration  of  focus  for  photograph- 
ing can  be  done  at  the  eye-piece  by  a  suitable  arrangement  provided  for  that  purpose. 

Osmond.  —  Osmond  used  a  Nachet  microscope  of  the  ordinary  type  connected 
with  a  vertical  camera  and  a  prism  illuminator.  He  writes,  however,  that  special 
metallurgical  microscopes  "are  certainly  to  be  preferred."  In  Osmond's  opinion  the 
vertical  is  very  much  superior  to  the  horizontal  camera  for  studying  metals. 

Nachet.  —  Nachet  of  Paris  constructs  the  metallurgical  microscope  (Fig.  35) . 
The  vertical  illuminator  carries  a  tube  provided  with  an  iris  diaphragm.  The  stand 
is  to  be  used  in  the  vertical  position  only.  The  stage  has  a  coarse  vertical  adjust- 
ment. A  similar  microscope  is  made  with  mechanical  stage  provided  with  both 
coarse  and  fine  adjustments. 

The  prism  illuminator  (Fig.  36)  designed  by  Guillemin  is  made  by  Nachet.     A 


APPENDIX    I  —  MANIPULATIONS   AND   APPARATUS 


23 


lateral  as  well  as  a  slight  tilting  motion  may  be  imparted  to  the  prism  through  the 
milled  heads  B  and  C. 


Fig.  32.  —  Martcns-Zeiss  metallurgical  microscope  and  camera. 


Fig.      33.  —  Martens     ball-jointed 
microscope. 

Nachet's  illuminating  objectives  have  been  described  and  illustrated  (Apparatus 
for  the  Metallographic  Laboratory,  page  17). 

Cornu-Charpy.  —  The  arrangement  shown  in  Figure  37  was  used  by  Charpy.  The 
vertical  illuminator  G  consists  of  four  thin  glass  plates  placed  at  an  angle  of  45  deg. 


24 


APPENDIX    I  —  MANIPULATIONS   AND   APPARATUS 


immediately  below  the  eye-piece  and  it  receives  the  light  reflected  by  the  totally 
reflecting  prism  P.  This  prism  is  so  mounted  that  it  can  rotate  freely  around  the 
axis  of  the  microscope  and  also  around  the  axis  GP  of  the  tube  to  which  it  is  at- 
tached, thus  making  it  possible  to  receive  upon  it  the  light  proceeding  from  a  source 
of  light  placed  anywhere. 


Fig.  34.  —  Rosenhain  metallurgical  microscope. 

Watson  and  Sons.  —  The  metallurgical  microscope  (Fig.  38)  was  constructed 
in  1904  by  Watson  and  Sons  of  London.  The  stage  is  provided  with  both  coarse  and 
fine  adjustments.  The  same  makers  following  Martens  construct  the  horizontal 
metallurgical  microscope  (Fig.  39).  The  plain  glass  vertical  illuminator  (Fig.  40) 
provided  with  iris  diaphragm  is  also  made  by  Watson  and  Sons. 


APPENDIX    I  —  MANIPULATIONS  AND  APPARATUS 


25 


Fig.  35.  —  Nachet  metallurgical  microscope. 


Fig.  37.  —  Cornu-Charpy   metallurgical 
microscope. 


Fig.  36.  —  Guillomin-Nachet 
prism  illuminator. 


26 


APPENDIX    I  —  MANIPULATIONS   AND   APPARATUS 


Fig.  38.  —  Metallurgical  microscope  constructed  by 
Watson  and  Sons. 


Fig.  39.  —  Horizontal  metallurgical  microscope  constructed  by 
Watson  and  Sons. 


APPENDIX    I  —  MANIPULATIONS  AND   APPARATUS 


27 


It.  and  ./.  Beck.  —  In  1904  R.  and  J.  Beck  of  London  constructed  the  prism  ver- 
tical illuminator  shown  in  Figure  41.     The  device  is  fitted  with  an  iris  diaphragm 


Fig.  40.  —  Watson  and  Sons 
vortical  illuminator. 


Fig.  41.  —  Bock  prism 
illuminator. 


Fig.  42.  —  Beck  surface  microscope. 


Fig.  43.  —  Metallurgical  microscope.     (Queen  and  Co.) 

beneath  the  prism  for  cutting  off  outside  light,  and  a  plate  of  stops  so  arranged  that 
the  position  of  the  beam  of  light  impinging  on  the  prism  can  be  varied  until  parallel 
light  of  the  right  angle  is  obtained.  The  same  makers  construct  the  instrument 


28 


APPENDIX    I  —  MANIPULATIONS   AND   APPARATUS 


shown  in  Figure  42  for  the  examination  of  large  metallic  surfaces.  The  Rosenhain 
microscope  described  in  these  pages  is  likewise  made  by  R.  and  J.  Beck. 

Queen  and  Co.  —  Queen  and  Co.  of  Philadelphia  at  one  time  (1898)  placed  on  the 
market  the  microscope  and  camera  shown  in  Figure  43.  The  camera  could  be  tilted 
on  one  side  for  ocular  examination.  The  same  makers  now  construct  the  microscope 
shown  in  Figure  44. 

Arthur  H.  Thomas  Co.  —  Arthur  H.  Thomas  Co.  of   Philadelphia   are   offering 


J1 


Fig.  44.  —  Metallurgical  microscope.     (Queen 
and  Co.) 

for  sale  an  illuminator  designed  by  Wirt  Tassin  (Fig.  45).  A  condensing  lens  and 
acetylene  burner  are  attached  to  the  vertical  illuminator. 

F.  Koristka.  —  The  prism  vertical  illuminator  (Fig.  46)  was  described  by  F. 
Koristka  of  Milan  in  1905.  An  iris  diaphragm  placed  in  front  of  the  prism  controls 
the  light  which  it  receives.  By  pulling  out  the  arm  carrying  the  prism  the  latter  may 
be  removed  from  the  field. 

Ph.  Pellin.  —  The  Le  Chatelier  inverted  microscope  is  constructed  by  Ph.  Pellin 
of  Paris.  The  same  makers  also  manufacture  a  portable  microscopic  outfit  designed 


APPENDIX    I  —  MANIPULATIONS   AND   APPARATUS 


29 


by  Guillet  (Trousse  de  Metallographie).    It  includes  a  small  electric  motor  for  polish- 
irg,  a  vertical  microscope  so  constructed  that  it  can  be  fastened  to  any  object  it  is 


Fig.  45.  —  Microscope  and  camera  with  Tassin 
illuminator  attached. 


Fig.  40.  —  Koristka  prism  illuminator. 

desired  to  examine,  files,  emery-papers,  etching  reagents,  etc.     All  parts  are  com- 
pactly placed  in  a  carrying  case. 


30  APPENDIX    I  —  MANIPULATIONS   AND   APPARATUS 

Carl  Zeiss.  —  The  instruments  used  by  Martens  and  Heyn  already  described  in 
these  pages  are  constructed  by  Carl  Zeiss  of  Jena.  The  prism  vertical  illuminator 
made  by  the  same  firm  has  been  described  and  illustrated  in  the  introductory  chapter 
on  Apparatus. 

Spencer  Lens  Co.  —  The  Spencer  Lens  Co.  of  Buffalo,  N.  Y.,  manufacture  a 
vertical  metallurgical  microscope  with  movable  stage. 

Bausch  and  Lomb  Optical  Co.  —  The  microscopes  and  accessories  used  and  de- 
signed by  the  author  and  fully  described  in  these  .pages  are  manufactured  by  the 
Bausch  and  Lomb  Optical  Co.  of  Rochester,  N.  Y. 


APPENDIX   II 

REPORT    OF    COMMITTEE    53    OF    THE    INTERNATIONAL    ASSOCIATION    FOR 

TESTING   MATERIALS 

ON   THE  NOMENCLATURE  OF  THE  MICROSCOPIC  SUBSTANCES 
AND  STRUCTURES  OF  STEEL  AND   CAST  IRON 

Presented  by  the  Chairman  H.  M.  HOWE  and  the  Secretary  of  the  Committee  ALBERT  SAUVEUB 
at  the  VIth  Congress,  New  York,  September,  1912 

The  Committee  for  studying  this  problem  is  constituted  as  follows: 

Professor  H.  M.  HOWE,  Chairman,  New  York. 

Professor  ALBERT  SAUVEUR,  Secretary,  Cambridge,  Mass. 

Members:  F.  OSMOND,  Paris;  Dr.  H.  C.  H.  CARPENTER,  Manchester;  Prof.  W. 
CAMPBELL,  New  York;  Prof.  C.  BENEDICKS,  Stockholm;  Prof.  F.  Wiisx,  Aachen; 
Prof.  A.  STANSFIELD,  Montreal;  Dr.  J.  E.  STEAD,  Middlesbrough;  Prof.  L.  GUILLET, 
Paris;  Prof.  E.  HEYN,  Berlin-Lichterfelde;  Dr.  W.  ROSENHAIN,  Teddington. 

I.    GENERAL   PLAN 

We  first  enumerate  the  substances  of  such  importance  as  to  warrant  it,  indicating 
roughly  their  constitution,  and  then  define  and  describe  certain  of  them. 

The  conditions  which  we  meet  a^e  (1)  that  we  need  definitions  on  which  all  can 
agree;  and  this  implies  that  they  must  be  free  from  all  contentious  matter  and  be 
based  on  what  all  admit  to  be  true;  (2)  that  the  reader  must  needs  know  the  current 
theories  as  to  the  constitution  of  these  substances,  and  these  theories  are  necessarily 
contentious.  We  meet  these  conditions  by  the  plan  of  giving  (1)  the  Name  which  we 
recommend  for  general  use,  followed  immediately  in  parentheses  by  the  other  names 
used  widely  enough  to  justify  recording  them;  (2)  the  Definition  proper,  based  on  an 
undisputed  quality,  e.g.  that  of  austenite,  which  we  base  on  its  being  an  iron-carbon 
solid  solution,  purposely  omitting  all  reference  to  the  precise  nature  of  solvent  and 
solute;  and  (3)  Constitution,  etc.,  etc.,  in  which  we  give  the  current  theories  as  to  the 
nature  of  solvent  and  solute  and  appropriate  descriptive  matter. 

The  distinction  between  these  three  parts  should  be  understood.  (1)  The  Names 
actually  used  are  matters  of  record  and  indisputable.  (2)  The  Definitions  are  matters 
of  convention  or  treaty,  binding  on  the  contracting  parties,  though  subject  to  de- 
nouncement, preferably  based  on  some  determinable  property  of  the  thing  defined  as 
distinguished  from  any  theory  as  to  its  nature,  or  if  necessarily  based  on  any  theory 
it  should  be  a  theory  which  is  universally  accepted.  It  is  a  matter  purely  of  conven- 
tion and  general  convenience  what  individual  property  of  the  thing  defined  shall 
form  the  basis  of  the  definition.  The  name  and  the  definition  should  endure  perma- 
nently, except  in  the  case  of  a  definition  based  on  an  accepted  theory,  which  must  be 

1 


2       APPENDIX  II  —  NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS 

changed  if  the  theory  should  later  be  disproved.  (3)  Theories  and  Descriptions  are 
not  matters  of  agreement  or  convention  but  dependent  on  observation,  and  therefore 
always  subject  to  be  changed  by  new  discoveries.  They  are  temporary  in  their  nature 
:is  distinguished  from  the  names  and  definitions  which  should  be  fixed,  at  least  rela- 
tively. 

This  case  of  austenite  illustrates  the  advantage  of  non-indicative  names.  The 
names  which  we  propose  to  displace,  "gamma  iron"  and  "mixed  crystals,"  imply 
definite  theories  as  to  the  nature  of  austenite,  and  hence  might  have  to  be  abandoned 
in  case  those  theories  were  later  disproved.  The  name  "austenite"  implies  nothing, 
like  mineralogical  names  in  general,  and  hence  is  stable  in  itself.  Our  infant  branch 
of  science  may  well  learn  from  its  elder  sister,  which  has  tried  and  proved  the  advan- 
tage of  this  non-indicative  naming. 

In  those  cases  in  which  a  name  has  been  used  in  more  than  one  sense  we  advise 
the  retention  of  one  and  the  abandonment  of  the  others,  having  obtained  the  consent 
of  the  proposers  of  such  names  for  their  abandonment. 

Many  whose  judgment  we  respect  object  to  our  including  certain  of  the  less  used 
names,  e.g.  from  i  to  n  in  our  list,  holding  them  either  to  be  confusing  or  to  be  needless. 

It  is  true  that  several  names  (hardenite,  martensite,  sorbite,  etc.),  have  been  used 
with  various  meanings,  and  hence  confusingly,  in  spite  of  which  most  of  them  should 
Lie  retained,  each  with  a  single  sharp-cut  definition,  because  they  are  so  useful. 

As  regards  the  alleged  needlessness  of  certain  names  it  is  for  each  writer  to  decide 
whether  he  does  or  does  not  need  names  with  nice  shades  of  meaning,  such  as  osmon- 
dite  and  troosto-sorbite.  Those  who  look  only  at  the  general  outlines  and  not  at  the 
details  have  no  right  to  forbid  the  workers  in  detail  from  having  and  using  words 
fitting  their  work;  nor  have  those  whose  needs  are  satisfied  by  the  three  primary 
colors  a  right  to  forbid  painters,  dyers,  weavers,  and  others  from  naming  the  many 
shades  with  which  they  are  concerned.  Like  the  lexicographer  we  must  seive  the 
reader  by  explaining  those  words  which  he  will  meet,  whether  we  individually  use  or 
condemn  them.  We  feel  that  we  have  exhausted  our  powers  in  cautioning  writers 
that  certain  words  are  rare  and  not  likely  to  be  understood  by  most  readers,  or  are 
improper  for  any  reason,  and  in  urging  the  complete  abandonment  of  those  with- 
drawn by  their  proposers. 

Needless  words  will  die  a  natural  death;  needed  ones  we  cannot  kill.  The  good 
we  might  do  in  hastening  the  death  of  the  moribund  by  omitting  them  from  this  re- 
port is  less  than  the  good  we  do  by  teaching  their  meaning  to  those  who  will  meet 
them  in  ante-mortem  print.  These  readers  have  rights.  We  serve  no  class,  but  the 
whole. 

Illustrations.  —  At  the  end  of  the  several  descriptions  the  reader  is  referred  to 
good  illustrations  in  Osmond  and  Stead's  "Microscopic  Analysis  of  Metals,"  Griffin 
&  Co.,  London,  1904. 

II.    LIST   OF    MICROSCOPIC   SUBSTANCES 

The  microscopic  substances  here  described  consist  of 

1.  Meiarals,  true  phases,  like  the  minerals  of  nature.  These  are  either  elements, 
definite  chemical  compounds,  or  solid  solutions  and  hence  consisting  of  definite  sub- 
stances in  varying  proportions.  These  include  austenite,  ferrite,  cementite,  and 
graphite. 


APPENDIX  II  —  NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS       3 

2.  Aggregates,  like  the  petrographic  entities  as  distinguished  from  the  true  minerals. 
These  mixtures  may  he  in  definite  proportions,  i.e.  eutectic,  or  eutectoid  mixtures 
(ledeburite,  pearlite,  steadite),  or  in  indefinite  proportions  (troostite,  sorbite).  Those 
aggregates  which  are  important  for  any  reason  are  here  described. 

(Many  true  minerals,  such  as  mica,  felspar,  and  hornblende,  are  divisible  into 
several  different  species  so  that  these  true  mineral  names  may  be  either  generic  or 
specific.  These  genera  and  species  are  definite  chemical  compounds  in  which  one 
element  may  replace  another.  Other  minerals,  such  as  obsidian,  are  solid  solutions 
in  varying  proportions,  and  in  these  also  one  element  may  replace  another.  Metarals 
like  minerals  differ  from  aggregates  in  being  severally  chemically  homogeneous.) 

These  two  classes  may  be  cross  classified  into: 

(A)  The  iron-carbon  series,  which  come  into  being  in  cooling  and  heating. 

(B)  The  important  impurities,  manganese  sulphide,  ferrous  sulphide,  slag,  etc. 

(C)  Other  substances. 

The  most  prominent  members  of  the  iron-carbon  series  are : 

I.  molten  iron,  metaral,  molten  solution,  but  hardly  a  microscopic  constituent; 

II.  the  components  which  form  in  its  solidification: 

(a)  austenite,  solid  solution  of  carbon  or  iron  carbide  in  iron,  metaral, 
(6)  cementite,  definite  metaral,  Fe3C, 
(c)  graphite,  definite  metaral,  C; 

III.  the  transition  substances  which  form  through  the  transformation  of  austenite 
during  cooling: 

(W)  martensite,  metaral  of  variable  constitution;  its  nature  is  in  dispute; 
(c)  troostite,  indefinite  aggregate,  uncoagulated  mixture, 

(/)  sorbite,  indefinite  aggregate,  chiefly  uncoagulated  pearlite  plus  ferrite  or  ce- 
mentite ; 

IV.  products1  of  the  transformation  of  austenite: 
(g)  ferrite, 

(/«)  pearlite. 

This  transformation  may  also  yield  cementite  and  graphite  as  end  products  in 
addition  to  those  under  b  and  c. 

In  addition  to  the  above,  the  names  of  which  are  universally  recognized  and  in 
general  use,  the  following  names  have  been  used  more  or  less: 

(i)  ledeburite  (Wiist),  definite  aggregate,  the  austenite-cementite  eutectic; 

(j)  ferronite  (Benedicks),  hypothetical  definite  metaral,  /3  iron  containing  about 
0.27  per  cent  of  carbon; 

(/,•)  steadite  (Sauveur),  definite  aggregate,  the  iron-phosphorus  eutectic  (rare); 

and  three  transition  stages  in  the  transformation  of  austenite,  viz. : 

(/)  hardenite  (Arnold),  collective  name  for  the  austenite  and  martensite  of  eutec- 
toid composition; 

(m)  osmondite  (Heyn),  boundary  stage  between  troostite  and  sorbite; 

(n)  troosto-sorbite  (Kourbatoff) ,  indefinite  aggregate,  the  troostite  and  the  sorbite 
which  lie  near  the  boundary  which  separates  these  two  aggregates  (obsolescent). 

1  In  hypo-cut ectoid  steels  these  habitually  play  the  part  of  end  products,  though  according  to 
the  belief  of  most  the  true  end  of  the  transformation  is  not  reached  till  the  whole  has  changed  into 
a  conglomerate  of  ferrite  plus  graphite. 


4      APPENDIX  II  —  NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS 


III.     DEFINITIONS    AND    DESCRIPTIONS 

Carbon-iron  Equilibrium  Diagram,  Figure  1.  —  Under  the  several  substances  about 
to  be  described  an  indication  will  be  given  of  the  parts  of  the  carbon-iron  equilibrium 
diagram  Figure  I  to  which  they  severally  correspond. 

Austenite,  Osmond  (Fr.  Austenite,  Ger.  Austenit,  called  also  mixed  crystals  and 
gamma  iron.  Up  to  the  year  1900  often  called  martensite  and  wrongly  sometimes 
still  so  called).  Metaral  of  variable  composition. 

Definition.  —  The  iron-carbon  solid  solution  as  it  exists  above  the  transformation 


1500 


KtOO- 

1300- 

1200- 

£1100- 

21000- 

£900- 

0 

H 

800- 
M 

700- 
600- 


500- 


1. 
Molten    Iron 

(Per  Fondu) 


Molben   Iron 
(Per  Fondu) 


flusbenite  +  Cementibe 


a  A. 

oc-Ferrite 

+ 

Pearl  ite 


8.B. 

Cementibe 

Pear  I  i  be 


K 


1  2  3  4  5 

Carbon  per  cent 

Fig.  1.  — A,:  The  line  PSK  is  often  called  "A,".    A3:  The  line  COS  is  often  called  "A3' 
this  name  is  sometimes  applied  to  the  line  SE. 


and 


range  or  as  preserved  with  but  moderate  transformation  at  lower  temperatures,  e.g. 
by  rapid  cooling,  or  by  the  presence  of  retarding  elements  (Mn,  Ni,  etc.),  as  in  12  per 
cent  manganese  steel  and  25  per  cent  nickel  steel. 

Constitution  and  Composition.  —  A  solid  solution  of  carbon  or  iron  carbide  (prob- 
ably Fe3C)  and  gamma  iron,  normally  stable  only  above  the  line  PSK  of  the  carbon- 
iron  diagram.  It  may  have  any  carbon  content  up  to  saturation  as  shown  by  the  line 
SE,  viz.:  about  0.90  per  cent  at  S  (about  725  deg.  C.)  to  1.7  per  cent  at  E  (about 
1130  deg.).  The  theory  that  the  iron  and  the  carbide  or  carbon,  instead  of  being  dis- 
solved in  each  other,  are  dissolved  in  a  third  substance,  the  solution  of  eutectoid  com- 


APPENDIX  II  —  NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS      5 

position  (Fc24C,  called  hardenite)  is  not  in  accord  with  the  generally  accepted  theory 
of  the  constitution  of  solutions,  and  is  not  entertained  widely  or  by  any  member  of 
this  committee. 

Crystallization.  —  Isometric.  The  idiomorphic  vug  crystals  are  octahedra  much 
elongated  by  parallel  growth.  The  etched  sections  show  much  twinning.  (Osmond 
and  most  authorities.)  Le  Chatelier  believes  it  to  be  rhombohedral.  Cleavage 
octahedral. 

Varieties  and  Genesis.  —  (l)  Primary  austenite  formed  in  the  solidification  of  carbon 
steel  and  hypo-eutectic  cast  iron;  (2)  eutectic  austenite,  interstratified  with  eutectic 
cementite,  making  up  the  eutectic  formed  at  the  end  of  the  solidification  of  steel  con- 
taining more  than  about  1.7  per  cent  of  carbon,  and  of  all  cast  iron. 

Equilibrium.  —  It  is  normal  and  in  equilibrium  in  Region  4,  and  also  associated 
with  beta  iron  in  Region  6,  with  a  iron  in  Region  7,  and  with  cementite  in  Region  5. 
It  should  normally  transform  into  pearlite  with  either  ferrite  or  cementite  on  cooling 
past  AI  into  Region  8. 

Transformation.  —  In  cooling  slowly  through  the  transformation  range,  Ar3  -  Ari, 
austenite  shifts  its  carbon  content  spontaneously  through  generating  pro-eutectoid 
ferrite  or  cementite,  to  the  eutectoid  ratio,  about  0.90  per  cent,  and  then  transforms 
with  increase  of  volume  at  Ari  into  pearlite.  q.v.,  with  which  the  ejected  ferrite  or 
cementite  remains  mixed.  Rapid  cooling  and  the  presence  of  carbon,  manganese,  and 
nickel  obstruct  this  transformation,  (l)  retarding  it,  and  (2)  lowering  the  temperature  at 
which  it  actually  occurs,  and  in  addition  (3)  manganese  and  nickel  lower  the  temperature 
at  which  in  equilibrium  it  is  due.  Hence,  by  combining  these  four  obstructing  agents 
in  proper  proportions  the  transformation  may  be  arrested  at  any  of  the  intermediate 
stages,  martensite,  troostite,  or  sorbite,1  q.v.,  and  if  arrested  in  an  earlier  stage  it 
can  be  brought  to  any  later  desired  stage  by  a  regulated  reheating  or  "tempering." 
For  instance,  though  a  very  rapid  cooling  in  the  absence  of  the  three  obstructing  ele- 
ments checks  the  transformation  but  little  and  only  temporarily,  yet  if  aided  by  the 
presence  of  a  little  carbon  it  arrests  the  transformation  wholly  in  the  martensite 
stage;  and  in  the  presence  of  about  1.50  per  cent  of  carbon  such  cooling  retains  about 
half  the  austenite  so  little  altered  that  it  is  "considerably"  softer  than  the  usually 
darker  needles  of  the  surrounding  martensite,  with  which  it  contrasts  sharply.  Again, 
either  (a)  about  12  per  cent  of  manganese  plus  1  per  cent  of  carbon,  or  (6)  25  per  cent 
of  nickel,  lower  and  obstruct  the  transformation  to  such  a  degree  that  austenite  per- 
sists in  the  cold  apparently  unaltered,  even  through  a  slow  cooling.  (Hadfield's  man- 
ganese steel  and  25  per  cent  nickel  steel,  manganiferous  and  nickeliferous  austenite 
respectively.) 

Occurrence.  —  When  alone  (12  per  cent  manganese  and  25  per  cent  nickel  steel 
and  Maurer's  2  per  cent  carbon  plus  2  per  cent  manganese  austenite)  polyhedra,  often 
coarse,  much  twinned  at  least  in  the  presence  of  martensite,  and  readily  developing 
slip  bands.  In  hardened  high-carbon  steel  it  forms  a  ground  mass  pierced  by  zigzag 
needles  and  lances  of  martensite. 

Etching.  —  All  the  common  reagents  darken  it  much  more  than  cementite,  less 

1  Though  the  transformation  can  be  arrested  in  such  a  way  as  to  leave  the  whole  of  the  steel 
in  the  condition  of  martensite,  it  is  doubted  by  some  whether  it  can  be  so  arrested  as  to  leave  the 
whole  of  it  in  any  of  the  other  transition  stages.  Troostite  and  sorbite  caused  by  such  arrest  are 
habitually  mixed,  troostite  with  martensite  or  sorbite  or  both,  and  sorbite  with  pearlite  or  troostite 
or  both. 


6       APPENDIX  II— NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS 

than  troostite  or  sorbite,  and  usually  less,  though  sometimes  more,  than  martcnsite, 
which  is  recognized  by  its  zigzag  shape  and  needle  structure.  With  ferrite  and  pearlite 
it  is  never  associated. 

Physical  Properties.  —  Maurer's  austenite  of  2  per  cent  manganese  plus  2  per 
cent  carbon  is  but  little  harder  than  soft  iron,  and  25  per  cent  nickel  steel  and  Had- 
field's  manganese  steel  are  but  moderately  hard.  Yet  as  usually  preserved  in  hardened 
high  carbon  steel,  the  hardness  of  austenite  does  not  fall  very  far  short  of  that  of  the 
accompanying  martensite,  probably  because  partly  transformed  in  cooling.  (Os- 
mond's words  are  that  it  is  "considerably"  softer  than  that  martensite.) 

Specific  Magnetism.  —  Very  slight  unless  perhaps  in  intense  fields.  In  Hadfield's 
manganese  steel  and  25  per  cent  nickel  steel,  very  ductile. 

Illustrations.  —  "Microscopic  Analysis  of  Metals,"  Figures  20,  50,  and  51  on 
pp.  39,  100,  and  101. 

Cementite  (Sorby,  "intensely  hard  compound";  Ger.  Cementit,  Fr.  Cementite; 
Arnold,  crystallized  normal  carbide).  Definite  mctaral. 

Definition.  —  Tri-ferrous  carbide,  FeaC.  The  name  is  extended  by  some  writers 
so  as  to  include  tri-carbides  in  which  part  of  the  iron  is  replaced  by  manganese  or 
other  elements.  Such  carbides  may  be  called  " manganiferous  cementite,"  etc. 

Occurrence.  —  (a)  Pearlitic  as  a  component  of  pearlite,  q.v.;  (b)  eutectic; 
(c)  primary  or  pro-eutectic;  (d)  pro-eutectoid;  (e)  that  liberated  by  the  splitting  up  of 
the  eutectic  or  of  pearlite;  and  (/)  uncoagulated  in  sorbite,  troosite,  and  perhaps  mar- 
tensite. (c),  (d),  and  (e)  are  grouped  together  as  "free"  or  "massive." 

Primary  cementite  is  generated  in  cooling  through  Region  3;  eutectic  cementite 
on  cooling  past  the  line  EBD;  pro-eutectoid  cementite  in  cooling  through  Region  5; 
pearlitic  cementite  on  cooling  past  the  line  PSK,  or  AI.  Though  the  several  varieties 
of  cementite  are  generally  held  to  be  all  metastable,  tending  to  break  up  into  graphite 
plus  either  austenite  above  AI  or  ferrite  below  AI,  yet  they  have  a  considerable  and 
often  great  degree  of  persistence.  The  graphitizing  tendency  is  completely  checked 
in  the  cold  but  increases  with  the  temperature  and  with  the  proportion  of  carbon 
and  of  silicon  present,  and  is  opposed  by  the  presence  of  manganese. 

Crystallization.  —  Orthorhombic,  in  plates. 

Structure.  —  (a)  Pearlitic,  in  parallel  unintersecting  plates  alternating  with  plates 
of  ferrite;  (b)  eutectic,  plates  forming  a  network  filled  with  a  fine  conglomerate 
of  pearlite  with  or  without  pro-eutectoid  cementite;  (c)  primary,  in  manganiferous 
white  cast  iron,  etc.,  in  rhombohedral  plates;  (d)  in  hyper-eutectoid  steel,  pro-eutec- 
toid cementite  forms  primarily  a  network  enclosing  meshes  of  pearlite  through  which 
cementite  plates  or  spines  sometimes  shoot  if  the  network  is  coarse;  (e)  cementite 
liberated  from  pearlite  merges  with  any  neighboring  cementite;  (/)  the  structure  of 
uncoagulated  cementite  cannot  be  made  out.  On  long  heating  the  pro-eutectoid 
and  pearlitic  cementite  spheroidize  slowly,  and  neighboring  particles  merge;  (o)  in 
white  irons  rich  in  phosphorus  in  flat  plates  embedded  in  iron-carbon-phosphorus 
eutectic. 

Etching,  etc.  —  After  polishing  stands  in  relief.  Brilliant  white  after  etching  with 
dilute  hydrochloric  or  picric  acid;  darkened  by  boiling  with  solution  of  sodium  picrate 
in  excess  of  sodium  hydrate. 

Physical  Properties.  —  Hardest  component  of  steel.  Hardness  =  6  of  Mohs  scale. 
Scratches  glass  and  felspar  but  not  quartz;  very  brittle.  Specific  magnetism  about 
two  thirds  that  of  pure  iron. 


APPENDIX  II  —  NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS       7 

Illustrations.  —  "Microscopic  Analysis  of  Metals,"  Figures  42  and  43  on 
pp.  84,  85. 

Martensite  (Fr.  Martensite,  Ger.  Martensit).     Metaral.    Its  nature  is  in  dispute. 

Definition.  —  The  early  stage  in  the  transformation  of  austenite  characterized  by 
needle  structure  and  great  hardness,  as  in  hardened  high-carbon  steel. 

Constitution.  —  I.  (Osmond  and  others.)  A  solid  solution  like  austenite,  q.v.,  ex- 
cept that  the  iron  is  partly  beta,  whence  its  hardness,  and  partly  alpha,  whence  its 
magnetism  in  mild  fields.  II.  (Le  Chatelier.)  The  same  except  that  its  iron  is  essen- 
tially alpha,  and  the  hardness  due  to  the  state  of  solid  solution.  III.  (Arnold.)  A  spe- 
cial structural  condition  of  his  "hardenite"  (austenite);  not  \videlyjield.  IV.  A  solid 
solution  in  gamma  iron.  V.  (Benedicks.)  The  same  as  I,  except  that  the  iron  is  wholly 
beta  and  that  beta  iron  consists  of  alpha  iron  containing  a  definite  quantity  of  gamma 
iron  in  solution. 

Equilibrium.  —  It  is  not  in  equilibrium  in  any  part  of  the  diagram,  but  represents 
a  metastable  condition  in  which  the  metal  is  caught  during  rapid  cooling,  in  transit 
between  the  austenite  condition  stable  above  the  line  AI  and  the  condition  of  ferrite 
plus  cementite  into  which  the  steel  habitually  passes  on  cooling  slowly  past  the 
line  AI. 

Occurrence.  —  The  chief  constituent  of  hardened  carbon  tool  steels,  and  of  medium 
nickel  and  manganese  steels.  In  still  less  fully  transformed  steels  (1.50  per  cent 
carbon  steel  rapidly  quenched,  etc.)  it  is  associated  with  austenite;  in  more  fully 
transformed  ones  (lower  carbon  steels  hardened,  high  carbon  steels  oil  hardened,  or 
water  hardened  and  slightly  tempered,  or  hardened  thick  pieces  even  of  high  carbon 
steel)  it  is  associated  with  troostite,  and  with  some  pro-eutectoid  ferrite  or  cementite, 
q.v.,  in  hypo-  and  hyper-eutectoid  steels  respectively.  In  tempering  it  first  changes 
into  troostite;  at  350  deg. -400  deg.  it  passes  through  the  stage  of  osmondite;  at 
higher  temperatures  it  changes  into  sorbite;  and  at  700  deg.  into  granular  pearlite. 
On  lipating  into  the  transformation  range  this  changes  into  austenite,  which  on  cool- 
ing again  yields  lamellar  pearlite. 

Characteristic  specimens  are  had  by  quenching  bars  1  cm.  square  of  eutectoid 
steel,  i.e.  steel  containing  about  0.9  per  cent  of  carbon,  in  cold  water  from  800  deg.  C. 
(1472  deg.  F.). 

Structure.  —  When  alone,  habitually  in  flat  plates  made  up  of  intersecting  needles 
parallel  to  the  sides  of  a  triangle.  When  mixed  with  austenite,  zigzag  needles,  lances, 
and  shafts. 

If  produced  by  quenching  after  heating  to  735  deg.  C.,  it  consists  of  minute  crystal- 
lites resembling  the  globulites  of  Vogelsang,  which  are  rarely  arranged  in  triangular 
order.  At  times  so  fine  as  to  suggest  being  amorphous. 

Etching.  —  With  picric  acid,  iodine  or  very  dilute  nitric  acid  etches  usually  darker 
than  austenite,  but  sometimes  lighter,  always  darker  than  ferrite  and  cementite,  but 
always  lighter  than  troostite. 

Illustrations.  —  "Microscopic  Analysis  of  Metals,"  Figure  19  on  p.  38,  Figure  52 
on  p.  102. 

Ferrite    (Fr.  Ferrite,  Ger.  Ferrit).     Definite  metaral. 

Definition.  —  Free  alpha  iron. 

Composition.  —  Nearly  pure  iron.  It  may  contain  a  little  phosphorus  and 
silicon  but  its  carbon  content,  if  any,  is  always  small,  at  the  most  not  more  than  0.05 
per  cent,  and  perhaps  never  as  much  as  0.02  per  cent. 


8       APPENDIX  II  — NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS 

Occurrence.  —  (a)  Pearlitic  as  a  component  of  pearlite,  q.v.;  (6)  pro-eutectoid 
ferrite  generated  in  slow  cooling  through  the  transformation  range;  (c)  that  segre- 
gated from  pearlite,  i.e.  set  free  by  the  splitting  up  of  pearlite,  especially  in  low  car- 
bon steel;  (d)  uncoagulated  as  in  sorbite,  and  probably  troostite.  (6)  and  (c)  are 
classed  together  as  free  or  massive. 

Thus  ferrite  is  normal  and  stable  in  regions  7  and  8. 

Crystallization.  —  Isometric,  in  cubes  or  octahedra. 

Structure.  —  (a)  Pearlitic  ferrite,  unintersecting  parallel  plates  alternating  with 
plates  of  cementite;  (6)  pro-eutectoid  ferrite  in  low  carbon  steel  forms  irregular  poly- 
gons, each  with  uniform  internal  orientation.  In  higher  carbon  steel  after  moderately 
slow  cooling,  especially  in  the  presence  of  manganese,  it  forms  a  network  enclosing 
meshes  of  pearlite.  In  slower  cooling  this  network  is  replaced  by  irregular  grains 
separated  by  pearlite;  (c)  the  ferrite  set  free  by  the  splitting  up  of  pearlite  merges 
with  the  pro-eutectoid  ferrite,  if  any;  (d)  the  structure  of  the  ferrite  in  sorbite,  etc., 
cannot  be  made  out. 

Etching.  —  Dilute  alcoholic  nitric  or  picric  acid  on  light  etching  leaves  the 
ferrite  grains  white  with  junctions  which  look  dark.  Deeper  etching,  by  Heyn's 
reagent  or  its  equivalent,  reveals  the  different  orientation  of  the  crystals  or. grains, 
(a)  as  square  figures  parallel  to  the  direction  of  the  etched  surface,  (6)  as  plates  which 
dip  at  varying  angles  and  become  dark  or  bright  when  the  specimen  is  rotated  under 
oblique  illumination.  Still  deeper  etching  reveals  the  component  cubes  (etching 
figures,  Atzfiguren),  at  least  if  the  surface  is  nearly  parallel  to  the  cube  faces. 

Physical  Properties.  —  Soft;  relatively  weak  (tenacity  about  40,000  Ibs.,  per 
sq.  in.);  very  ductile;  strongly  f erro-magnetic ;  coercitive  force  very  small. 

Grain  Size.  —  For  important  purposes  (1)  etch  deeply  enough,  e.g.  with  copper- 
ammonium' chloride,  to  reveal  clearly  the  junctions  of  the  grains;  (2)  count  on  a  photo- 
graph of  small  magnification  the  number  of  grains  in  a  measured  field  so  drawn  as  to 
exclude  fragments  of  grains;  after  (3)  determining  the  true  grain  boundaries  by  ex- 
amination under  high  powers  (Heyn's  method).  Deep  nitric  acid  etching  is  inaccurate, 
because  an  apparent  grain  boundary  may  contain  several  grains. 

Illustrations.  —  "Microscopic  Analysis  of  Metals,"  Figures  41,  56  on  pp.  79,  116. 

Osmondite  (Fr.  Osmondite,  Ger.  Osmondit). 

Definition.  —  That  stage  in  the  transformation  of  austenite  at  which  the  solubility 
in  dilute  sulphuric  acid  reaches  its  maximum  rapidity.  Arbitrarily  taken  as  the 
boundary  between  troostite  and  sorbite. 

Earlier  Definition.  —  Defined  by  the  Vth  Congress  as  having  the  "maximum  sol- 
ubility in  acids  and  by  a  maximum  coloration  under  the  action  of  acid  metallographic 
reagents."  The  present  definition  is  confined  to  maximum  rapidity  of  dissolving, 
because  we  do  not  yet  know  that  this  in  all  cases  co-exists  with  the  maximum  depth 
of  coloration,  and  in  any  case  in  which  these  two  should  not  co-exist,  the  old  defini- 
tion does  not  decide  which  is  true  osmondite. 

Constitution.  —  The  following  hypotheses  have  been  suggested,  none  of  which 
has  firm  experimental  foundation:  (1)  A  solid  solution  of  carbon  or  an  iron  carbide 
in  alpha  iron.  (2)  The  colloidal  system  of  Benedicks  in  its  purity,  troostite  being  this 
system  while  forming  at  the  expense  of  martensite,  and  sorbite,  being  this  system 
coagulating  and  passing  into  pearlite.  (3)  The  stage  of  maximum  purity  of  amor- 
phous alpha  iron  on  the  way  to  crystallizing  into  ferrite. 

Occurrence.  —  Hardened  carbon  steel  of  about  1  per  cent  of  carbon  when  reheated 


APPENDIX  II  — NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS       9 

(tempered)  to  350^00  deg.  C.  passes  through  the  stage  of  troostite  to  that  of 
osmondite,  and  on  higher  heating  to  that  of  sorbite.  What  variation  if  any  from  this 
temperature  is  needed  to  bring  hardened  steel  of  other  carbon  content  to  the  osmond- 
ite stage  is  not  known.  In  that  it  represents  a  true  boundary  state  between  troostite 
and  sorbite  it  differs  in  meaning  from  troosto-sorbite,  which  embraces  both  the  troost- 
ite and  the  sorbite  which  lie  near  this  boundary.  Indeed  osmondite  has  sometimes 
been  used  in  this  looser  sense.  Writers  are  cautioned  that,  however  useful  these  terms 
may  prove  for  making  these  nice  discriminations,  they  are  not  likely  to  be  familiar 
to  general  readers. 

Etching.  —  According  to  Heyn  it  differs  from  troostite  and  sorbite  in  being  that 
stage  in  tempering  which  colors  darkest  on  etching  with  alcoholtc^iydrochloric  acid. 

The  present  definition  and  description  of  osmondite  should  displace  previous  ones, 
because  they  have  the  express  approval  of  Professor  Heyn,  the  proposer  of  the  name, 
and  M.  Osmond  himself. 

Ferronite  (Fr. Ferronite,  Ger.  Ferronit)  (Benedicks).   Hypothetical  definite  metaral. 

Definition.  —  Solid  solution  of  about  0.27  per  cent  of  carbon  in  beta  iron. 

Occurrence  (hypothetical) .  —  In  slowly  cooled  steels  and  cast  iron  containing 
0.50  per  cent  of  combined  carbon  or  more,  that  which  is  generally  believed  to  be  fer- 
rite,  whether  pearlitic  or  free,  is  supposed  by  Benedicks  to  be  ferronite. 

Hardenite  (Fr.  Hardenite,  Ger.  Hardenit). 

Definition.  —  Collective  name  for  austenite  and  martensite  of  eutectoid  composi- 
tion. It  includes  such  steel  (1)  when  above  the  transformation  range,  and  (2)  when 
hardened  by  rapid  cooling. 

Observations.  —  On  the  generally  accepted  theory  that  austenite  is  a  solid  solution 
of  carbon  or  an  iron  carbide  in  iron,  hardenite  is  the  solution  of  the  lowest  transforma- 
tion temperature,  i.e.  the  eutectoid.  The  theory  that  instead  it  is  a  definite  chemical 
compound,  Fe2,iC,  is  considered  under  Austenite.  Its  proposer  includes  under 
hardenite  both  eutectoid  (0.90  per  cent  carbon)  austenite  when  above  the  transforma- 
tion range  and  the  martensite  into  which  that  austenite  shifts  in  rapid  cooling  (hard- 
ening) . 

Other  Meanings.  —  Originally  (Howe,  1888)  collective  name  for  aastenite  and 
martensite  of  any  composition  in  carbon  steel.  Osmond  (1897),  austenite  saturated 
with  carbon.  Both  these  meanings  are  withdrawn  by  their  proposers. 

Pearlite  (Sorby's  "pearly  constituent."  At  first  written  "pearlyte"  Fr.  Perlite, 
(!er.  Perlit).  Aggregate. 

Definition.  —  The  iron-carbon  eutectoid,  consisting  of  alternate  masses  of  ferrite 
and  cementite. 

Constitution  and  Composition.  —  A  conglomerate  of  about  6  parts  of  ferrite  to  1  of 
cementite.  When  pure,  contains  about  0.90  per  cent  of  carbon,  99.10  per  cent  of 
iron. 

Occurrence.  —  Results  from  the  completion  of  the  transformation  of  austenite 
brought  spontaneously  to  the  eutectoid  carbon  content,  and  hence  occurs  in  all 
carbon  steels  and  cast  iron  containing  combined  carbon  and  cooled  slowly  through 
the  transformation  range,  or  held  at  temperatures  in  or  but  slightly  below  that  range, 
long  enough  to  enable  the  ferrite  and  cementite  to  coagulate  into  a  mass  microscopic- 
ally resoluble.  Hence  it  is  the  normal  constituent  in  Region  8.  Its  ferrite  is  stable 
but  its  cementite  is  metastable  and  tends  to  transform  into  ferrite  and  graphite. 

Varieties  and  Structure. — Because  pearlite  is  formed  by  the  coagulation  of  the 


10    APPENDIX  II  —  NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS 

ferrite  and  cementite  initially  formed  as  the  irresoluble  emulsion,  sorbite,  (Arnold's 
sorbitic  pearlite)  there  are  the  indefinitely  bounded  stages  of  sorbitic  pearlite  (Arnold's 
normal  pearlite),  i.e.  barely  resoluble  pearlite,  in  the  border-land  between  sorbite  and 
laminated  pearlite;  granular  pearlite,  in  which  the  cementite  forms  fine  globules  in  a 
matrix  of  ferrite;  and  laminated  or  lamellar  pearlite,  consisting  of  fine,  clearly  defined, 
non-intersecting,  parallel  lamellae  alternately  of  ferrite  and  cementite.  The  name 
granular  pearlite  was  first  used  by  Sauveur  to  represent  what  is  now  called  sorbite. 
This  meaning  has  been  withdrawn. 

An  objection  to  Arnold's  name  "normal  pearlite"  is  that  it  is  likely  to  mislead. 
"Normal"  here  apparently  refers  to  arising  under  normal  conditions  of  cooling,  but 
(1)  it  rather  suggests  structure  normal  for  pearlite,  which  surely  is  the  lamination 
characteristic  of  eutectics  in  general,  and  (2)  the  general  reader  has  no  clue  as  to  what 
conditions  of  cooling  are  here  called  normal.  Many  readers  are  not  manufacturers, 
and  even  in  manufacture  itself  air  cooling  is  normal  for  one  branch  and  extremely 
slow  furnace  cooling  for  another.  Arnold  calls  troostite  "troostitic  pearlite"  and 
sorbite  "sorbitic  pearlite."  This  is  contrary  to  general  usage,  which  restricts  pearlite 
to  microscopically  resoluble  masses. 

Etching.  —  After  etching  with  dilute  alcoholic  nitric  or  picric  acid  it  is  darker  than 
ferrite  or  cementite  but  lighter  than  sorbite  and  troostite.  A  magnification  of  at 
least  250  diameters  is  usually  needed  for  resolving  it  into  its  lamellae,  though  the 
pearlite  of  blister  steel  can  often  be  resolved  with  a  magnification  of  25  diameters. 
The  more  rapidly  pearlite  is  formed,  the  higher  the  magnification  needed  for  re- 
solving it. 

Illustrations.  —  Lamellar  pearlite.  Osmond  and  Stead,  "Microscopic  Analysis," 
Figure  11  on  p.  19,  Granular  pearlite,  idem,  Figure  18  on  p.  36;  Heyn  and  Bauer, 
"Stahl  und  Eisen,"  1906,  Figure  14,  opposite  p.  785. 

Graphite    (Ger.  Graphit,  Fr.  Graphite).     Definite  metaral. 

Definition.  —  The  free  elemental  carbon  which  occurs  in  iron  and  steel. 

Composition.  —  Probably  pure  carbon,  identical  with  native  graphite. 

Genesis.  —  Derived  in  large  part,  and  according  to  Gosrens  wholly,  from  the  de- 
composition of  solid  cementite.  Others  hold  that  its  formation  as  kish  may  be  from 
solution  in  the  molten  metal,  and  that  part  of  the  formation  of  temper  graphite  may 
be  from  elemental  carbon  dissolved  in  austenite.  It  is  the  stable  form  of  carbon  in 
all  parts  of  the  diagram. 

Occurrence.  —  (1)  as  kish,  flakes  which  rise  to  the  surface  of  molten  cast  iron  and 
usually  escape  thence; 

(2)  as  thin  plates,  usually  curved,  e.g.  in  gray  cast  iron,  representing  carbon  which 
has  separated  during  great  mobility,  i.e.  near  the  melting  range; 

(3)  as  temper  graphite  (Ger.  Temperkohle,  Ledebur)  pulverulent  carbon  which 
separates  from  cementite  and  austenite,  especially  in  the  annealing  process  for  mak- 
ing malleablized  castings. 

Graphite  and  ferrite  are  sometimes  associated  in  a  way  which  suggests  strongly 
that  they  represent  a  graphite-austenite  eutectic.  But  the  existence  of  such  a  true 
eutectic  is  doubted  by  most  writers. 

Properties.  —  Hexagonal.  H.  1-2.  Gr.  2.255.  Streak  black  and  shining,  luster 
metallic;  macroscopic  color,  iron  black  to  dark  steel  gray,  but  always  black  when  seen 
in  polished  sections  of  iron  or  steel  under  the  microscope;  opaque;  sectile;  soils  paper; 
flexible;  feel,  greasy. 


APPENDIX  II  —  NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS      11 

Troostite  (Fr.  Troostite,  Ger.  Trcostit).  Probably  agrregate.  (Arnold,  troostitic 
pearlite.) 

Definition.  —  In  the  transformation  of  austenite,  the  stage  following  martensite 
and  preceding  sorbite  (and  osmondite  if  this  stage  is  recognized). 

Constitution  and  Composition.  —  An  uncoagulatecl  conglomerate  of  the  transition 
stages.  The  degree  of  completeness  of  the  transformation  represented  by  it  is  not 
definitely  known  and  probably  varies  widely.  Osmond  and  most  others  believe  that 
the  transformation,  while  generally  far  advanced,  yet  falls  materially  short  of  comple- 
tion; but  Benedicks  and  Arnold  (9)  believe  that  it  is  complete.  The  former  belief 
that  it  is  a  definite  phase,  e.g.  a  solid  solution  of  carbon  or  an  iron  carbide  in  either  /3 
or  7  iron,  is  abandoned.  Its  carbon  content  like  that  of  austeTiitc  and  martensite 
varies  widely. 

Occurrence.  —  It  arises  either  on  reheating  hardened  (e.g.  martensitic  steel)  to 
slightly  below  400  deg.,  or  on  cooling  through  the  transformation  range  at  an  inter- 
mediate rate,  e.g.  in  small  pieces  of  steel  when  quenched  in  oil,  or  quenched  in  water 
from  the  middle  of  the  transformation  range,  or  in  the  middle  of  larger  pieces  quenched 
in  water  from  above  the  transformation  range.  With  slightly  farther  reheating  it 
changes  into  sorbite;  with  higher  heating  into  sorbitic  pearlite,  then  slowly  into  granular 
pearlite,  and  probably  indirectly  into  lamellar  pearlite.  It  occurs  in  irregular,  fine- 
granular  or  almost  amorphous  areas,  colored  darker  by  the  common  etching  reagents 
than  the  martensite  or  sorbite  accompanying  it.  A  further  common  means  of  dis- 
tinguishing it  from  sorbite  is  that  it  is  habitually  associated  with  martensite,  whereas 
sorbite  is  habitually  associated  with  pearlite. 

Areas  near  the  boundary  between  troostite  and  sorbite  are  sometimes  called 
troosto-sorbite. 

Properties.  —  Hardness,  intermediate  between  that  of  the  martensitic  and  the 
pearlitic  state  corresponding  to  the  carbon  content  of  the  specimen.  In  general  the 
hardness  increases,  the  elastic  limit  rises,  and  the  ductility  decreases,  as  the  carbon 
content  increases.  Its  ductility  is  increased  rapidly  and  its  hardness  and  elastic  limit 
lowered  rapidly  by  further  tempering,  which  affects  it  much  more  markedly  than 
sorbite. 

Sorbite   (Fr.  Sorbite,  Ger.  Sorbit).    Aggregate.    (Arnold,  sorbitic  pearlite.) 

Definition.  —  In  the  transformation  of  austenite,  the  stage  following  troostite 
and  osmondite  if  the  stage  is  recognized,  and  preceding  pearlite. 

Constitution  and  Composition.  —  Most  writers  believe  that  it  is  essentially  an  un- 
coagulated  conglomerate  of  irresoluble  pearlite  with  ferrite  in  hypo-  and  cementite 
in  hyper-eutectoid  steels  respectively,  but  that  it  often  contains  some  incompletely 
transformed  matter. 

Occurrence.  —  The  transformation  can  be  brought  to  the  sorbitic  stage  (1)  by  re- 
heating hardened  steel  to  a  little  above  400  deg.,  but  not  to  700  deg.  at  which  tem- 
perature it  coagulates  into  granular  pearlite;  (2)  by  quenching  small  pieces  of  steel 
in  oil  or  molten  lead  or  even  by  air  cooling  them;  (3)  by  quenching  in  water  from  just 
above  the  bottom  of  the  transformation  range,  Ari.  Sorbite  is  ill-defined,  almost  amor- 
phous, and  is  colored  lighter  than  troostite  but  darker  than  pearlite  by  the  usual 
etching  reagents.  It  differs  further  from  troostite  in  being  softer  for  given  carbon 
content,  and  usually  in  being  associated  with  pearlite  instead  of  martensite,  and 
from  pearlite  in  being  irresoluble  into  separate  particles  of  ferrite  and  cementite. 

As  sorbite  is  essentially  a  mode  of  aggregation  it  cannot  properly  be  represented 


12      APPENDIX  II  —  NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS 

on  the  equilibrium  diagram.  Its  components  at  all  times  tend  to  coagulate  into 
pearlite,  yet  it  remains  in  its  uncoagulated  state  at  all  temperatures  below  400  dog. 

Properties.  —  Though  slightly  less  ductile  than  pearlitic  steel  for  given  carbon 
content,  its  tenacity  and  elastic  limit  are  so  high  that  a  higher  combination  of  these 
three  properties  can  be  had  in  sorbitic  than  in  pearlitic  steels  by  selecting  a  carbon 
content  slightly  lower  than  would  be  used  for  a  pearlitic  steel.  Hence  the  use  of 
sorbitic  steels,  e.g.  first  hardened  and  then  annealed  cautiously,  for  structural  pur- 
poses needing  the  best  quality. 

Manganese  Sulphide  (Fr.  Sulphur  de  Manganese,  Ger.  Schwefelmangan),  MnS 
(Arnold  and  Waterhouse).  Metaral. 

Occurrence,  etc.  —  Sulphur  combines  with  the  manganese  present  in  preference  to 
the  iron,  forming  pale  dove  or  slate  gray  masses,  rounded  in  castings,  elongated  in 
forgings. 

Ferrous  Sulphide   (Fr.  Sulphure  de  Fer,  Ger.  Schwefeleisen),  FeS.     Metaral. 

Occurrence.  —  The  sulphur  not  taken  up  by  the  manganese  forms  ferrous  sulphide, 
FeS,  which,  probably  associated  in  part  with  iron  as  an  Fe-FeS  eutectic,  forms  by 
preference  more  or  less  continuous  membranes  surrounding  the  grains  of  pearlite. 
Color,  yellow  or  pale  brown. 

Sulphur  Prints.  —  When  silk  impregnated  with  mercuric  chloride  and  hydrochloric 
acid  (Heyn's  and  Bauer's  method)  or  bromide  paper  moistened  with  sulphuric  acid 
(Baumann's  method)  is  pressed  on  polished  steel,  the  position  of  the  sulphur-bearing 
areas,  whether  of  FeS  or  MnS,  records  itself  by  the  local  blackening  which  the  evolved 
H2S  causes.  Phosphorus  bearing  areas  also  blacken  Baumann's  bromide  paper. 

MISCELLANEOUS 

Eutectoid,  Saturated,  etc.  —  The  iron-carbon  eutectoid  is  pearlite.  Steel  with  more 
carbon  than  pearlite  is  called  hyper-eutectoid,  that  with  less  is  called  hypo-eutectoid. 
Arnold's  names  "saturated,"  "unsaturated,"  and  "supersaturated,"  for  eutectoid, 
hypo-eutectoid,  and  hyper-eutectoid  steel  respectively,  have  considerable  industrial 
use  in  English-speaking  countries,  but  are  avoided  by  most  scientific  writers  on  the 
ground  that  they  are  misleading,  because,  e.g.  there  is  only  one  specific  temperature, 
AI,  at  which  eutectoid  steel  is  actually  saturated,  and,  if  any  other  temperature  is  in 
mind,  that  steel  is  not  saturated.  Above  AI  it  is  clearly  undersaturated. 

The  objection  to  the  names  sorbite,  troostite,  martensite,  and  austenite,  that 
each  of  them  covers  steel  of  a  wide  range  of  carbon  content,  is  to  be  dismissed  because 
a  like  objection  applies  with  equal  force  to  every  generic  name  in  existence. 


The  theoretical  matter  in  this  report  is  given  solely  for  exposition  and  the  com- 
mittee disclaims  the  intent  to  impose  any  theory.  This  report  is  offered  for  adop- 
tion subject  to  this  disclaimer  on  the  ground  that  the  adoption  of  theories  is  beyond 
the  powers  of  a  Congress. 


INDEX 

The  Roman  numerals  refer  to  the  numbers  of  the  lessons,  the  letter  A  to  the  chapter 
on  "  Apparatus  for  the  Metallographic  Laboratory." 


A,  AT,  Ac,  Ar3,  Ac3,  Ar3.2,  Ac3.2,  Ar3.2.i,  Ac3.2.i,  Arcm,  Accm.     See  critical 

points,  notation 

Allotrimorphic  crystals,  definition  of,  I,  2 
Allotropic  theory  of  the  hardening  of  steel,  XV,  2 
Allotropy,  definition  of,  II,  4 

of  cementite,  VIII,  9 
iron,  II,  4;  VIII,  1,  14 
sulphur,  II,  6 

Alloy  steels.     See  special  steels 
Alloys,  constitution  of,  XXII,  1  to  21 

,  fusibility  curves  of,  XXII,  5  to  21 
,  microstructure  of,  XXII,  5  to  21 

of  iron  and  carbon,  equilibrium  diagram  of,  XXIII,  12  to  21 
,  fusibility  curves  of,  XXIII,  1  to  21 
,  phase  rule  applied  to,  XXIV,  6  to  8 
,  structural  composition  immediately  after  soli- 
dification of,  XXIII,  3 
,  phase  rule  applied  to,  XXIV,  3  to  8 
,  solidification  of,  XXII,  3  to  21 
,  structural  composition  of,  XXII,  15  to  20 
whose  component  metals  form  solid  solutions,  solidification  and 

constitution  of,  XXII,  3  to  9 
are  insoluble  in  each  other  in  the  solid 
state,  solidification  and  constitution 
of,  XXII,  9  to  17 
partially  soluble  in  each  other  in 
the  solid  state,  solidification  and 
constitution  of,  XXII,  17  to  21 
Alpha  iron,  VIII,  1,  14;  IX,  8 

,  crystallization  of,  II,  7 
,  description  of,  II,  6 

theory  of  the  hardening  of  steel,  XV,  4 

Alumina  powder  for  polishing,  preparation  of,  Appendix  I,  2 
Ammonium  oxalate  etching,  V,  7 
Anhedrons.     See  allotrimorphic  crystals 
Annealing,  air  cooling  in,  XII,  4 
,  cooling  in,  XII,  2 
,  double  treatment  in,  XII,  8 

for  malleablizing  cast  iron,  XXI,  3  to  7 
,  furnace  cooling  in,  XII,  4 
,  heating  for,  XII,  1 

,  influence  of  maximum  temperature  in,  XII,  5 
,  influence  of  time  at  maximum  temperature  in,  XII,  6 
,  nature  of  operation,  XII,  1 
1 


INDEX 

Annealing  of  steel,  XII,  1  to  30 

,  oil  and  water  quenching  in,  XII,  6 
,  purpose  of,  XII,  1 

,  rate  of  cooling  vs.  carbon  content  in,  XII,  3 
size  of  objects  in,  XII,  3 
steel  castings,  XII,  13 
temperatures  for  steel,  XII,  2 
Arnold  on  the  hardening  of  steel,  XV,  1,  4 
Arnold's  view  of  the  nature  of  martensite,  XIII,  10 

troostite,  XIII,  12 
Austenite,  crystallization  of,  X,  1  to  10 

,  definition,  description,  occurrence,  and  structure  of,  XIII,  3  to  9 
,  growth  above  the  critical  range  of,  XII,  20 
,  Osmond's  test  showing  relative  softness  of,  XIII,  7 
,  production  by  Maurer  of,  XIII,  5 
Osmond  of,  XIII,  4 
Robin  of,  XIII,  5 
,  relative  softness  of,  XIII,  7 
,  saturated,  XXIII,  2 

Austenitic  and  pearlitic  structures,  relation  between,  XII,  21 
special  steels,  XVII,  7 
steel,  tempering  of,  XIV,  3 


Belaiew  on  the  structure  of  steel  and  of  meteorites,  X,  6  to  10 
Benedicks'  equilibrium  diagram  of  iron-carbon  alloys,  XXIII,  19 

view  of  the  nature  of  troostite,  XIII,  12 
Beta  iron,  VIII,  1,  11,  14;  IX,  8 
,  crystallization  of,  II,  7 
,  description  of,  II,  6 

theory  of  the  hardening  of  steel,  XV,  2 
Binary  alloys.     See  alloys. 
Bivariant  equilibrium,  definition  of,  XXIV,  3 
Black  heart  castings,  XXI,  4 

,  annealing  for,  XXI,  5 
Brass,  twinnings  in,  II,  7 
Brittleness,  intercrystalline,  XII,  27 
,  intergranular,  XII,  27 

of  low  carbon  steel,  XII,  26 
Burnt  steel,  production  and  structure  of,  XII,  17  to  20 


C 

Cameras,  A,  22  to  28 
Carbide  steel,  XVII,  1 

Carbon,  condition  of,  in  hardened  and  tempered  steel,  XIV,  8 
,  hardening  and  combined  in  steel,  XIV,  8 
in  pearlite,  V,  8 
in  steel,  IV,  3 
temper,  XXI,  1 

theory  of  the  hardening  of  steel,  XV,  1,  4 
Carpenter  and  Heeling's  cooling  curves  of  steels,  VII,  17 

determinations  of  the  critical  points,  VII,  8,  9 
equilibrium  diagram  of  iron-carbon  alloys,  XXIII,  18,  19 
Case  hardened  articles,  heat  treatment  of,  XVI,  6 

steel,  tempering  of,  XVI,  6 
hardening,  composition  of  iron  or  steel  subjected  to,  XVI,  1 


INDEX 

Case  hardening,  cooling  after,  XVI,  5 

,  distribution  of  carbon  after,  XVI,  2 

,  duration  of,  XVI,  2 

,  materials  used  for,  XVI,  3 

,  mechansim  of,  XVI,  5 

of  steel,  XVI,  1  to  6 
,  temperatures  for,  XVI,  1 

Cast  iron,  calculation  of  structural  composition  of,  XIX,  5,  10  to  13;  XX,  7  to  9 
,  chilled  castings  of,  XIX,  13 

,  constitution,  properties,  and  structure  of,  XIX,  1  to  13;  XX,  1  to  10 
containing  only  combined  carbon,  XIX,  3 
graphitic  carbon,  XIX,  1 

,  formation  of  combined  and  graphitic  carbon  in,  XIX,  1 
,  impurities  in,  XX,  1  to  10 

,  influence  and  occurrence  of  manganese  in,  XX,  2 

phosphorus  in,  XX,  2 
silicon  in,  XX,  1 
sulphur  in,  XX,  1 
,  malleable,  XXI,  1  to  8 

,  structural  composition  vs.  physical  properties  of,  XIX,  11 
steel,  structure  of,  X,  1  to  10 
Castings  suitable  for  malleablizing,  XXI,  2 
Cement  carbon,  definition  of,  XIV,  8 
Cementation.     See  case  hardening 

of  iron  and  steel,  XVI,  1  to  6 
Cementite,  allotropy  of,  VIII,  9 

,  definition  and  description  of,  IV,  5 

,  etching  of,  V,  7;  XIX,  4 

,  formation  of,  X,  4 

,  free,  definition  of,  V,  5 

,  graphitizing  of,  XII,  15;  XXI,  1;  XXIII,  7 

in  high  carbon  steel,  V,  4 
,  primary.     See  cementite,  pro-eutectic 
,  pro-eutectic,  XXIII,  5 
,  spheroidizing  of,  XII,  14 
Cementitic  special  steels,  XVII,  1,  8 
Charpy  and  Grenet  on  the  equilibrium  diagram  of  iron-carbon  alloys,  XXIII,  20 

on  the  hardening  of  steel,  XV,  1,  5 
Chilled  castings,  XIX,  13 
Chrome-nickel  steel,  XVIII,  16 
steel,  XVIII,  13  to  1,5 

,  uses  and  properties  of,  XVIII,  14 
-tungsten  steel.     See  high-speed  steel. 

Chromium,  influence  on  critical  points  of  iron  of,  XVIII,  13 
Cleavage,  definition  of,  I,  1 

brittlcness.     See  intercrystalline  brittleness,  XII,  27 
Cold  working,  crystalline  growth  after,  I,  8 

,  influence  on  structure  and  properties  of  steel  of,  XI,  8 
Colloidal  solution,  XIII,  12 
Combined  carbon  in  cast  iron,  XIX,  1,  3 
Components,  Bancroft's  definition  of,  XXIV,  3 
,  definition  of,  XXIV,  3 
,  Findlay's  definition  of,  XXIV,  3 
,  Howe's  definition  of,  XXIV,  3 
,  Mellor's  definition  of,  XXIV,  3 
Condensers,  A,  21 

Cooling  and  heating  curves  of  iron  and  steel,  VII,  10  to  19 
curves  of  pure  metals,  XXII,  1 


INDEX 

Copper,  microstructure  of,  I,  1 
Critical  points  and  crystallization,  IX,  2,  5 
dilatation,  IX,  1  to  3 
electrical  conductivity,  IX,  1 
magnetic  properties,  IX,  3 
in  high  carbon  (hypcr-cutcctoid)  steel,  VII,  7 
iron,  description  of,  II,  6 
medium  high  carbon  steel,  VII,  6;  VIII,  4 
pure  iron,  VII,  5,  10;  VIII,  1 
very  low  carbon  steel,  VII,  6;  VIII,  3 
,  Carpenter  and  Reeling's  determination  of,  VII,  8 
,  causes  of,  VIII,  1  to  16 
,  definition  of,  VII,  1 
,  determination  of,  VII,  10 

,  graphical  representation  of  the  position  and  magnitude  of,  VII,  10 
,  heat  absorbed  or  evolved  at,  VII,  8 
,  influence  of  chemical  composition  on  position  of,  VII,  5 

speed  of  heating  and  cooling  on,  VII,  4 
,  instruction  for  detection  of,  VII,  19 
,  merging  of,  VII,  6,  7,  8 
,  minor,  VII,  8 
,  notation,  VII,  2 
,  occurrence  of,  VII,  1  to  20 
,  relation  between  structure  of  steel  and,  VIII,  12 
'    ,  their  effects,  IX,  1  to  8 

,  use  of  neutral  bodies  in  detecting,  VII,  14 
range.     See  critical  points, 
temperatures.     See  critical  points. 
Crystalline  grains.     See  grains 

growth  in  metals  on  annealing,  I,  7 

of  strained  ferrite,  XII,  23  to  26 
Crystallite  of  iron,  II,  5 
Crystallites,  definition  of,  I,  2 
Crystallization  and  critical  points,  IX,  2,  5 
,  cubic,  of  metals,  I,  4 
of  austenite,  X,  1  to  10 

iron,  II,  2 
,  process  of,  I,  1 

Crystallography,  systems  of,  I,  4 
Crystals,  allotrimorphic,  definition  of,  I,  2 
,  cubic,  of  iron,  II,  3,  4 
,  definition  of,  I,  1 
,  formation  of,  I,  1 
,  idiomorphic,  definition  of,  I,  2 
,  mixed.     See  mixed  crystals. 
Cubic  crystallization  of  iron,  II,  2 
metals,  I,  4 


Degrees  of  freedom,  definition  of,  XXIV,  2 
liberty.     See  degrees  of  freedom 
Desch's  types  of  cooling  curves,  VII,  18 
Dilatation  and  critical  points,  IX,  1  to  3 
Divariant  equilibrium.     See  bivariant  equilibrium 
Double  annealing  treatment,  XII,  8 
Ductility  of  steel,  structural  composition  vs.,  V,  17 


INDEX 


Edwards  on  high  speed  steel,  XVIII,  20 

the  hardening  of  steel,  XV,  1,  2 

Edwards'  view  as  to  the  nature  of  martensite,  XIII,  10 
Electric  arc  lamps,  A,  19  to  21 

furnaces,  A,  35 

Electrical  conductivity  and  critical  points,  IX,  1 
Electrolytic  iron,  microstructure  of,  II,  1 
Electromagnetic  stages,  A,  11 
Equilibrium,  bivariant,  definition  of,  XXIV,  3 
,  definition  of,  XXIV,  1 
diagram.     See  fusibility  curves 

of  iron-carbon  alloys,  XXIII,  12  to  21 

,  Benedicks'  diagram,  XXIII,  20 
,  Carpenter  and  Keeling's  diagram, 

XXIII,  19 

,  Roberts-Austen's  diagrams,  XXIII,  17 
,  Roozeboom's  diagram,  XXIII,  17 
,  Rosenhain's  diagram,  XXIII,  20 
,  the  author's  early  diagram,  XXIII,  16 
,  metastable,  definition  of,  XXIV,  2 
,  stable,  definition  of,  XXIV,  2 
,  univariant,  definition  of,  XXIV,  3 
,  unstable,  definition  of,  XXIV,  2 
,  unvariant,  definition  of,  XXIV,  3 
Etching,  III,  6;  Appendix  I,  10 

figures.     See  etching  pits 
of  cementite,  V,  7;  XIX,  4 
pits,  formation  of,  I,  4 

in  iron,  II,  3 

with  ammonium  oxalate,  V,  7 
nitric  acid,  III,  7 
picric  acid,  III,  6 
sodium  picrate,  V,  7 
Eutectic  alloys,  I,  5,  6 

,  constitution  and  occurrence  of,  XXII,  12  to  21 
,  definition  of,  XXII,  12 
,  iron-carbon,  XXIII,  2 
Eutectoid,  definition  of,  IV,  3 

steel,  definition  and  structure  of,  V,  4 
Ewing  and  Rosenhain,  straining  of  iron  by,  II,  11 

Ewing  and  Rosenhain's  theory  of  crystalline  growth  of  metals  on  annealing,  I,  7 
Eye-pieces,  A,  3 


Ferrite,  crystalline  growth  of,  XII,  23  to  26 
,  definition  of,  II,  4 
,  free,  IV,  4 

in  cast  iron,  XIX,  1  to  10 
low  carbon  steel,  IV,  2 
wrought  iron,  III,  1 
grains,  II,  1 

,  orientation  of,  II,  2 
Ferro-ferrite,  II,  4 
Fibers  in  wrought  iron,  III,  2 
Finishing  temperatures,  influence  on  the  structure  and  properties  of  steel  of,  XI,  3 


INDEX 

Free  cementite,  definition  of,  V,  5 

ferrite,  IV,  4 
Furnaces,  A,  35 
Fusibility  curves  of  alloys,  XXII,  5  to  21 

iron-carbon  alloys,  XXIII,  1  to  21 


Gamma  iron,  VIII,  1,  14;  IX,  8 

,  crystallization  of,  II,  7 
,  description  of,  II,  6 

theory  of  the  hardening  of  steel,  X7,  2 
,  twinning  in,  II,  7 
Ghost  lines  in  steel,  VI,  10 
Gold,  microstructure  of,  I,  1 
Grading  of  steel  vs.  its  carbon  content,  IV,  1 
Grain  refining  treatment,  XII,  8 
Grains,  crystalline  orientation  of,  I,  3 
,  ferrite,  II,  1 

,  orientation  of,  II,  2 
,  growth  of,  on  annealing,  I,  7 
of  metals,  definition  and  formation  of,  I,  3 

,  heterogeneousness  of,  I,  3 
Graphitic  carbon,  factors  influencing  formation  of,  XIX,  1 

in  cast  iron,  XIX,  1,  2,  3 
Graphitizing  of  cementite,  XII,  15;  XXI,  1;  XXIII,  7 

in  malleablizing  cast  iron,  XXI,  1 
Gray  cast  iron,  XIX,  8 

vs.  malleable  cast  iron,  XXI,  7 
Grenet  on  the  hardening  of  steel,  XV,  1,  5 
Guillaume  on  nickel  steel,  XVIII,  5 
Guillet  on  case  hardening,  XVI,  3,  4,  5 
chrome  steel,  XVIII,  14 
manganese  steel,  XVIII,  5 
nickel  steel,  XVIII,  1 
silicon  steel,  XVIII,  15 
the  hardening  of  steel,  XV,  1,  4 
tungsten  steel,  XVIII,  12 
vanadium  steel,  XVIII,  15,  17 
Guillet's  theory  of  special  steels,  XVII,  1 
Gutowsky  on  the  equilibrium  diagram  of  iron-carbon  alloys,  XXIII,  20 

H 

Hadfield  steel,  XVIII,  10 
Hard  castings,  XXI,  2 

Hardened  and  tempered  steel,  microstructure  of,  XIV,  7 
Hardening  and  tempering  in  one  operation,  XIII,  20;  XIV,  2 
carbon,  definition  of,  XIV,  8 

theory  of  the  hardening  of  steel,  XV,  4 
,  cooling  for,  XIII,  1 
,  heating  for,  XIII,  1 
of  steel,  XIII,  1  to  21 

,  theories  of,  XV,  1  to  7 
,  structural  changes  on,  XIII,  2 

theories  of  the  hardening  of  steel,  classification  of,  XV,  1 
Hardenite,  definition,  occurrence,  and  properties  of,  XIII,  15 
Heat  tinting,  Appendix  I,  11 


INDEX 

Heat  treatment  of  case  hardened  articles,  XVI,  6 
iron,  influence  of,  II,  10 
metals,  influence  of,  I,  7 

Heating  and  cooling  curves  of  iron  and  steel,  VII,  10  to  19 
Heraeus  electric  furnace,  A,  35 

Heyn  on  decrease  of  hardness  in  tempering,  XIV,  9 
heat  liberated  on  tempering  steel,  XIV,  9 
osinondite,  XIV,  6 

the  condition  of  carbon  in  hardened  and  tempered  steel,  XIV,  8 
equilibrium  diagram  of  iron-carbon  alloys,  XXIII,  20 
structure  of  hardened  and  tempered  steel,  XIV,  7 
High-speed  steel,  XVIII,  17  to  20 

,  composition  of,  XVIII,  18 
,  discovery  by  Taylor  and  White  of,  XVIII,  18 
,  microstructure  of,  XVIII,  18 
,  properties  of,  XVIII,  17 
,  theory  of,  XVIII,  18 
,  treatment  of,  XVIII,  17,  18 

Hot  working,  influence  on  structure  and  properties  of  steel  of,  XI,  1  to  7 
Howe  on  tempering  colors,  XIV,  1 

the  burning  of  steel,  XII,  17 
hardening  of  steel,  XV,  1 

Humfrey  and  llosenhain.  See  Rosenhain  and  Humfrey 
Hypcr-eutectoid  steel,  definition  and  structure  of,  V,  4 
Hypo-eutectoid  steel,  definition  and  structure  of,  V,  4 


Idiomorphic  crystals,  definition  of,  I,  2 
Illuminating  objectives,  A,  17 
Illumination  for  microscopical  work,  A,  14  to  22 
,  oblique,  A,  14  to  16 
,  vertical,  A,  14  to  18 
Illuminators,  vertical,  A,  14,  16  to  18 
Impurities  in  cast  iron,  XX,  1  to  10 

,  influence  on  iron  of,  II,  10 
in  metals,  influence  of,  I,  5 
steel,  VI,  1  to  12 

,  segregation  of,  VI,  10 
Ingot  iron,  II,  1 
Ingotism,  X,  n 

Intcrcrystalline  brittleness,  XII,  27 
Intergranular  brittleness,  XII,  27 
Invar  (nickel  steel),  XVIII,  5 
Inverted  microscope,  A,  28 
Iris  diaphragms,  A,  7 
Iron,  affinity  for  carbon  of,  XVI,  1 
,  allotropy  of,  II,  4;  VIII,  1,  14- 
,  alpha,  VIII,  1,  14;  IX,  8 

,  description  of,  II,  6 
,  beta,  VIII,  1,  11,  14;  IX,  8 

,  description  of,  II,  6 

-carbon  alloys,  equilibrium  diagram  of,  XXIII,  12  to  21 
,  fusibility  curves  of,  XXIII,  1  to  21 
,  phase  rule  applied  to,  XXIV,  6  to  8 
,  structural  composition  immediately  after  solidificat  ion 

of,  XXIII,  3 
eutectic,  XXIII,  2 


INDEX 

Iron,  cementation  of,  XVI,  1  to  6 

-cementite  fusibility  curve,  XXIII,  1 

,  cooling  and  heating  curves  of,  VII,  10  to  14 

,  critical  points  of,  VII,  5,  10;  VIII,  1 

crystallite,  II,  5 
,  crystallization  of,  II,  2 
,  cubic  crystals  of,  II,  3,  4 
,  electrolytic,  microstructure  of,  II,  1 
,  etching  in  hydrogen,  II,  10 

pits  in,  II,  3,  4 
,  gamma,  VIII,  1,  14;  IX,  8 

,  description  of,  II,  6 
-graphite  fusibility  curve,  XXIII,  7 
,  influence  of  chromium  on  critical  points  of,  XVIII,  13 
heat  treatment  of,  II,  10 
impurities  on,  II,  10 
mechanical  treatment  of,  II,  11 
nickel  on  dilatation  of,  XVIII,  5 
tungsten  on  critical  points  of,  XVIII,  12 
,  microstructure  of,  II,  1 

oxide  in  steel,  VI,  8 
,  slip  bands  in,  II,  11 
,  straining  of,  II,  10,  11 
sulphide  in  steel,  VI,  3 
Irreversible  steels,  XVIII,  2 
Isomorphous  mixtures,  definition  of,  I,  5 


Kourbatoff's  etching  to  color  cementite,  V,  7 
Kroll,  etching  of  pure  iron  in  hydrogen  by,  II,  9 


Le  Chatelier,  Andre,  on  the  hardening  of  steel,  XV,  1,  5 
Le  Chatelier  on  the  hardening  of  steel,  XV,  1,  4 

thermo-electric  pyrometer  for  the  determination  of  critical 

points,  VII,  10;  A,  30 

Le  Chatelier's  view  of  the  nature  of  martensite,  XIII,  10 
Ledebur's  temper  carbon,  XXI,  1 
Lieberkiihn,  A,  14 

Lights  for  microscopical  work,  A,  14  to  22 
Liquidus,  definition  of,  XXII,  4 

11 

Magnetic  properties  and  critical  points,  IX,  3 

specimen  holders,  A,  9,  11 
Malleable  cast  iron,  XXI,  1  to  8 

,  annealing  for  the  manufacture  of,  XXI,  3 

,  packing  materials  for  the  manufacture  of,  XXI,  3 

vs.  gray  cast  iron,  XXI,  7 
castings.     See  malleable  cast  iron 
Manganese  in  cast  iron,  influence  and  occurrence  of,  XX,  2 

steel,  VI,  5 
oxide  in  steel,  VI,  8 
steel,  XVIII,  5  to  12 

,  austenitic,  XVIII,  10 
,  martensitic,  XVIII,  10 


INDEX 

Manganese  steel,  pearlitic,  XVIII,  8 

,  properties  of  austenitic,  XVIII,  11 
,  treatment  of  austenitic,  XVIII,  11 
,  water-toughening  of,  XVIII,  1 1 
sulphide  in  steel,  VI,  3 
Marble,  twinnings  in,  II,  7 
Martensite,  Arnold's  view  as  to  the  nature  of,  XIII,  10 

,  definition,  description,  occurrence,  properties,   etching,   and 

structure  of,  XIII,  10 

,  Edwards'  view  as  to  the  nature  of,  XIII,  10 
,  Le  Chateh'er's  view  as  to  the  nature  of,  XIII,  10 
,  Osmond's  view  as  to  the  nature  of,  XIII,  10 
Martrnsitic  special  steels,  XVII,  7,  9 

steel,  tempering  of,  XIV,  5 
Matweieff's  etching  to  color  cementite,  V,  7 

method  of  etching  slag  in  wrought  iron,  III,  3 
Maurer,  production  of  austentite  by,  XIII,  5 
Mechanical  refining,  XI,  9 
stages,  A,  3,  12 
treatment  of  iron,  influence  of,  II,  11 

steel,  XI,  1  to  10 

Metalloscope,  universal,  A,  10  to  13,  28 
Metals,  cooling  curves  of,  XXII,  1 

,  crystalline  growth  on  annealing,  I,  7 

,  crystallization  of,  I,  1 

,  cubic  crystallization  of,  I,  4 

,  definition  and  formation  of  grains  of,  I,  3 

,  influence  of  heat  treatment,  I,  7 

mechanical  treatment  of,  I,  8 
,  latent  heat  of  solidification  of,  XXII,  2 
,  phase  rule  applied  to,  XXIV,  4 
,  solidification  of,  XXII,  1 
Metallic  alloys.     See  alloys 

,  constitution  of,  XXII,  1  to  21 
Metarals,  definition  of,  XIII,  18 
Metastable  equilibrium,  definition  of,  XXIV,  2 
Meteorites,  microstructure  of,  X,  6  to  10 
Microscopes  and  accessories,  A,  1  to  30;  Appendix  I,  16  to  30 

,  inverted,  A,  28 

Microstructure  of  cast  steel,  X,  1  to  10 
electrolytic  iron,  II,  1 
hardened  and  tempered  steel,  XIV,  7 
high  carbon  steel,  V,  4, 
sulphur  steel,  VI,  12 
vs.  low  phosphorus  steel,  VI,  11 
impure  gold,  I,  6 
low  carbon  steel,  IV,  2 
medium  high  carbon  steel,  V,  1 
meteorites,  X,  6  to  10 
oxidized  Bessemer  metal,  VI,  12 
pure  copper,  I,  1 
gold,  I,  1 
iron,  II,  1 
metals,  I,  1 
platinum,  I,  3 
worked  steel,  XI,  1  to  10 
wrought  iron,  III,  1,  2 
Mixed  crystals,  definition  of,  I,  6 


10  INDEX 

Monovariant  equilibrium.     See  univariant  equilibrium 

Mottled  cast  iron,  XIX,  10 

Mounting  samples,  Appendix  I,  12  to  15 

N 
Nachet  illuminating  objectives,  A,  17 

prism  vertical  illuminator,  A,  17 
Nernst  lamp,  A,  20 

Neutral  bodies  for  the  detection  of  critical  points,  VII,  14 
Nickel,  influence  of,  on  critical  points  of  iron,  XVIII,  2 

dilatation  of  iron,  XVIII,  5 
steel,  XVIII,  1  to  5 

,  austenitic,  XVIII,  5 
,  case  hardening  of,  XVIII,  4 
,  critical  points  of  commercial,  pearlitic,  XVIII,  2 
,  hardening  and  annealing  of,  XVIII,  4 
,  martensitic,  XVIII,  5 
,  pearlitic,  XVIII,  2 
,  properties  of  pearlitic,  XVIII,  3 
Nitric  :icid  etching,  III,  7 
Non-variant  equilibrium.     See  unvariant  equilibrium 

O 

Objectives,  A,  3 

Oblique  illumination,  A,  14  to  16 

Orientation  of  crystalline  grains,  definition  of,  I,  3 

ferrite  grains,  II,  2 
Osmond  on  the  hardening  of  steel,  XV,  1,  2 

,  production  of  austenite  by,  XIII,  4 
Osmond's  view  of  the  nature  of  martensite,  XIII,  10 
Osmondite,  definition,  description,  and  occurrence  of,  XIV,  5 
Oxalate  of  ammonium  etching,  V,  7 

P 

Parabolic  reflector,  A,  14 
Pearlite,  carbon  content  of,  V,  8 

,  definition  and  description  of,  IV,  3;  VIII,  7 
,  formation  of,  X,  1  to  10 
in  high  carbon  steel,  V,  4 
low  carbon  steel,  IV,  3 
,  varieties  of,  XII,  15 
Pearlitic  special  steels,  XVII,  6,  8 
Phase  rule  applied  to  alloys,  XXIV,  3  to  8 

iron-carbon  alloys,  XXIV.  6  to  8 
metals,  XXIV,  4 
,  definition  of,'  XXIV,  3 

,  enunciation  and  explanation  of,  XXIV,  1  to  4 
Phosphorus  in  cast  iron,  influence  and  occurrence  of,  XX,  2 

steel,  VI,  2 

Photomicrographic  cameras,  A,  22  to  28 
Photography.     See  photomicrography 
Photomicrography,  IV,  6 
Picrate  of  sodium  etching,  V,  7 
Picric  acid  etching,  III,  6 
Pits.     See  etching  pits 
Planes  of  cleavage.     See  cleavage 
Platinite  (nickel  steel),  XVIII,  5 
Platinum,  microstructure  of,  I,  3 


INDEX 

Point  of  rccalescence.     See  recalescence  point. 
Polishing,  III,  4;  Appendix  I,  1  to  9 

machines,  A,  28;  Appendix  I,  4  to  9 
Polyhedric  special  steels,  XVII,  1,  7 
Polymorphism.     See  allotropy 
Preserving  samples,  Appendix  I,  12 
Prism  vertical  illuminator,  A,  16,  17 
Pseudomorphism,  definition  of,  XIV,  7 
Pure  metals,  microstructure  of,  I,  1 
,  crystallization  of,  I,  1 
Pyrometer,  Le  Chatelier  thermo-electric,  for  the   determination  of  the 

critical  points,  VII,  10;  A,  30 
Pyrometers,  A,  30 

,  self-recording,  VII,  18;  A,  33,  34 

Q 

Quenching  in  annealing,  XII,  6 
Quaternary  steels,  XVII,  10.     See  also  special  steels 
vanadium  steels,  XVIII,  17 


Recalescence  point,  description  and  occurrence  of,  VII,  1 

Refining,  mechanical,  XI,  9 

Retardations.     See  critical  points 

Reversible  steels,  XVIII,  3 

Retention  theories  of  the  hardening  of  steel,  XV,  1 

Roberts-Austen  on  the  hardening  of  steel,  XV,  1,  2 

Roberts-Austen's  equilibrium  diagrams  of  iron-carbon  alloys,  XXIII,  17 

use  of  neutral  bodies  for  detecting  critical  points,  VII,  14;  A,  35 
Robin,  production  of  austenite  by,  XIII,  5 

Roozeboom's  equilibrium  diagram  of  iron-carbon  alloys,  XXIII,  17 
Rosenhain  and  Ewing.     See  Ewing  and  Rosenhain 

Humfrey,  straining  of  iron  by,  II,  10 
Rosenhain's  equilibrium  diagram  of  iron-carbon  alloys,  XXIII,  20 


Saladin  self-recording  pyrometer,  A,  33 
Saladin's  cooling  and  heating  curves  of  steels,  VII,  15,  16 
Segregation  of  impurities  in  steel,  VI,  10 
Self-hardening  steel,  XVIII,  13 

-recording  pyrometers,  VII,  18;  A,  33,  34 
Silicates  in  steel,  VI,  8 
Silicon  in  cast  iron,  influence  and  occurrence  of,  XX,  1 

steel,  VI,  1 
steel,  XVIII,  15 
Slag  in  wrought  iron,  III,  2 

,  composition  of,  III,  3 

Matweieff' s  method  of  etching,  III,  3 
,  microstructure  of,  III,  3 
Slip  bands,  description  and  production  of,  II,  10 

in  iron,  II,  11 

Sodium  picrate  etching,  V,  7 
Solid  solutions,  XXII,  4  to  9 

,  definition  of,  I,  5 
Solidus,  definition  of,  XXII,  4 
Solution  theories  of  the  hardening  of  steel,  XV,  2 


12  INDEX 

Sorbite,  definition,  description,  and  formation  of,  XI,  6;  XII,  5;  XIII,  13 
Sorby-Beck  parabolic  reflector,  A,  10 
Special  steels,  XVII,  1  to  10;  XVIII,  1  to  20 
,  austenitic,  XVII,  7,  9 
,  cementitic,  XVII,  1,  8,  9 
constitution,  properties,  treatment,  and  uses  of  most  important  types 

XVIII,  1  to  20 

,  definition  and  general  character  of,  XVII,  1  to  10 
,  influence  of  special  elements  on  position  of  critical  range  in,  XVII,  3 
,  martensitic,  XVII,  7,  9 
,  pearlitic,  XVII,  6,. 8 
,  polyhedric,  XVII,  1,  7,  9 
,  treatment  of,   XVII,  8 
Specimen  holders,  A,  7  to  9 
Spheroidizing  of  cementite,  XII,  14 
Stable  equilibrium,  definition  of,  XXIV,  2 
Stages,  electromagnetic,  A,  11 

,  mechanical,  A,  3,  12 
Stead  on  phosphorus  in  cast  iron,  XX,  3  to  7 

the  brittleness  of  low  carbon  steel,  XII,  26 

crystalline  growth  of  very  low  carbon  steel,  XII,  23 
Steadite,  definition  and  description  of,  XX,  3 
Stead's  brittleness,  XII,  28 
Steel,  annealing  of,  XII,  1  to  30 

,  temperatures  of,  XII,  2 
,  brittleness  of  low  carbon,  XII,  26 

,  calculation  of  structural  composition  of,  V,  8  to  12;  VI,  6,  7 
,  carbon  in,  IV,  3 
,  case  hardening  of,  XVI,  1  to  6 
castings,  annealing  of,  XII,  13 
,  causes  of  critical  points  in,  VIII,  1  to  16 
,  cementation  of,  XVI,  1  to  6 
,  chemical  tests  for  the  detection  of  sulphur  in,  VI,  4,  5 

vs.  structural  composition  of,  VI,  6 
,  chrome,  XVIII,  13  to  15 
-nickel,  XVIII,  16 
,  constitution,  properties,  treatment,  and  uses  of  most  important 

types  of  special,  XVIII,  1  to  20 
,  cooling  and  heating  curves  of,  VII,  10  to  19 
,  ductility  vs.  structural  composition  of,  V,  17 
,  eutectoid,  definition  and  structure  of,  V,  4 
,  effects  of  critical  points  in,  IX,  1  to  8 
,  formation  of  graphite  in  high  carbon,  XII,  15 
,  ghost  lines  in,  VI,  10 
,  hardening  of,  XIII,  1  to  21 
,  high  carbon,  cementite  in,  V,  4 

,  microstructure  of,  V,  4 
,  pearlite  in,  V,  4 
-speed,  XVIII,  17  to  20 

,  hyper-eutectoid,  definition  and  structure  of,  V,  4 
,  hypo-eutectoid,  definition  and  structure  of,  V,  4 
,  impurities  in,  VI,  1  to  12 
,  influence  of  cold  working  on  the  structure  and  properties  of,  XI,  8 

finishing  temperatures  on  the  structure  and  properties  of,  XI,  3 
hot  working  on  the  structure  and  properties  of,  XI,  1  to  7 
,  iron  oxide  in,  VI,  8 

sulphide  in,  VI,  3 
,  irreversible,  XVIII,  2 


INDEX  13 

Steel,  low  carbon,  etching  of,  IV,  6 
,  ferrite  in,  IV,  2 

,  microscopical  examination  of,  IV,  6 
,  microstructure  of,  IV,  2 
,  pearlite  in,  IV,  3 
,  vs.  wrought  iron,  IV,  1 
,  manganese,  XVIII,  5  to  12 
in,  VI,  5 
oxide  in,  VI,  8 
sulphide  in,  VI,  3 
,  maximum  strength  of,  V,  17 
,  mechanical  treatment  of,  XI,  1  to  10 
,  medium  high  carbon,  pearlite  in,  V,  1 

,  microstructure  of,  V,  1 
,  microstructure  of  high  vs.  low  phosphorus,  VI,  11 

sulphur,  VI,  12 
,  nickel,  XVIII,  1  to  5 
,  normal  structure  of,  IV,  1 
,  occurrence  of  critical  points  in,  VII,  1  to  20 

of  maximum  hardening  power,  XIII,  20 
,  phosphorus  in,  VI,  2 

,  physical  properties  of  constituents  of,  V,  14 
,  production  and  structure  of  burnt,  XII,  17  to  20 
,  relation  between  structure  and  critical  points  of,  VIII,  12 

above  and  below   the  critical  range  of, 

XII,  20 

,  reversible,  XVIII,  3 
,  segregation  of  impurities  in,  VI,  10 
,  self-hardening,  XVIII,  13 
,  silicates  in,  VI,  8 
,  silicon,  XVIII,  15 

in,  VI,  1 

,  special,  XVII,  1  to  10;  XVIII,  1  to  20 
,  structural  changes  on  cooling  in,  VIII,  5  to  16 
,  structure  of  cast,  X,  1  to  10 

worked,  XI,  1  to  10 
,  sulphur  in,  VI,  2 

,  tenacity  vs.  structural  composition  of,  V,  15 
,  tempering  of  hardened,  XIV,  1  to  10 
,  theories  of  hardening  of,  XV,  1  to  7 
,  tungsten,  XVIII,  12,  13,  17 
,  vanadium,  XVIII,  15,  17 

vs.  carbon  content,  grading  of,  IV,  1 
Straining,  crystalline  growth  after,  I,  7,  8 

of  iron,  II,  11 

Stress  theories  of  the  hardening  of  steel,  XV,  5 
Structural  composition  of  alloys,  XXII,  15  to  20 

cast   iron,   calculation  of,   XIX,   5,   10  to   13; 

XX,  7  to  9 

iron-carbon  alloys  immediately  after  solidifica- 
tion, XXIII,  3 

steel,  calculation  of,  V,  8  to  12;  VI,  6,  7 
Subcarbide  theory  of  the  hardening  of  steel,  XV,  4 
Sulphur,  allotropy  of,  II,  (i 

in  cast  iron,  influence  and  occurrence  of,  XX,  1 
steel,  VI,  2 

,  chemical  tests  for  the  detection  of,  VI,  4,  5 


INDEX 


Taylor  and  White's  discovery  of  high-speed  steel,  XVIII,  18 

Temper  carbon,  XXI,  1 

Temperatures  for  annealing  steel,  XII,  2 

Tempering  and  the  retention  theories  of  the  hardening  of  steel,  XV,  6 

stress  theory  of  the  hardening  of  steel,  XV,  6 
colors,  XIV,  1 

,  decrease  of  hardness  on,  XIV,  9 
,  explanation  of,  XIV,  2 
,  heat  liberated  on,  XIV,  9 
,  influence  of  rate  of  cooling  in,  XIV,  2 

time  in,  XIV,  1 
of  austenitic  steel,  XIV,  3 
case  hardened  steel,  XVI,  6 
hardened  steel,  XIV,  1  to  10 
martensitic  steel,  XIV,  5 
troostitic  steel,  XIV,  5 
temperatures,  XIV,  1 

Tenacity  of  steel,  structural  composition  vs.,  V,  15 
Ternary  steels,  XVII,  1.     See  also  special  steels 
Thermal  critical  points.     See  critical  points 

treatment.     See  heat  treatment 
Toughening  treatment,  XII,  8 
Transformation  points.     See  critical  points 
range.     See  critical  points 
Transition  constituents.     See  also  martensite,  troostite,  and  sorbite 

,  definition  and  formation  of,  XIII,  15,  17 
Troostite,  Arnold's  view  as  to  the  nature  of,  XIII,  12 

,  Benedicks'  view  as  to  the  nature  of,  XIII,  12 

,  definition,    description,    occurrence,  properties,   etching,   and 

structure  of,  XIII,  11 
Troostitic  steel,  tempering  of,  XIV,  5 
Troosto-sorbite,  XIII,  15 
Tschernoff  iron  crystallite,  II,  5 
Tungsten,  influence  on  the  critical  points  of  iron  of,  XVIII,  12 

steel,  XVIII,  12,  13 
Twinnings,  definition  of,  II,  7 
in  brass,  II,  7 

gamma  iron,  II,  7 
marble,  II,  7 
produced  by  pressure,  II,  7 

U 

Univariant  equilibrium,  definition  of,  XXIV,  3 
Universal  metalloscope,  A,  10  to  13,  28 
Unstable  equilibrium,  definition  of,  XXIV,  2 
Unvariant  equilibrium,  definition  of,  XXIV,  3 

V 

Vanadium  steel,  XVIII,  15,  17 
Vertical  illumination,  A,  14  to  16 

illuminators,  A,  14,  16  to  18 

W 

Water-toughening  of  manganese  steel,  XVIII,  11 
Welsbach  lamp,  A,  19 
Widmaustatten  structure,  X,  6 


INDEX  15 


White,  AJaunsel.     See  Taylor  and  White 
White  cast  iron,  XIX,  3 

heart  castings,  XXI,  4 

,  annealing  for,  XXI,  4 
Wrought  iron,  composition  of,  III,  1 
,  definition  of,  III,  1 
,  fibers  in,  III,  2 

,  microscopical  examination  of  structure  of,  III,  7 
,  microstructure  of,  III,  1,  2 
,  slag  in,  III,  2 
vs.  low  carbon  steel,  IV,  1 


Zeiss  prism  illuminator,  A,  17 


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